diff --git a/src/pkg/big/int.go b/src/pkg/big/int.go
index b96c387bf46..e66c34a8362 100755
--- a/src/pkg/big/int.go
+++ b/src/pkg/big/int.go
@@ -8,6 +8,7 @@ package big
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"rand"
 	"strings"
@@ -375,11 +376,48 @@ func (x *Int) Format(s fmt.State, ch int) {
 }
 
 
-// Scan is a support routine for fmt.Scanner. It accepts the formats
-// 'b' (binary), 'o' (octal), 'd' (decimal), 'x' (lowercase hexadecimal),
-// and 'X' (uppercase hexadecimal).
-func (x *Int) Scan(s fmt.ScanState, ch int) os.Error {
-	var base int
+// scan sets z to the integer value corresponding to the longest possible prefix
+// read from r representing a signed integer number in a given conversion base.
+// It returns z, the actual conversion base used, and an error, if any. In the
+// error case, the value of z is undefined. The syntax follows the syntax of
+// integer literals in Go.
+//
+// The base argument must be 0 or a value from 2 through MaxBase. If the base
+// is 0, the string prefix determines the actual conversion base. A prefix of
+// ``0x'' or ``0X'' selects base 16; the ``0'' prefix selects base 8, and a
+// ``0b'' or ``0B'' prefix selects base 2. Otherwise the selected base is 10.
+//
+func (z *Int) scan(r io.RuneScanner, base int) (*Int, int, os.Error) {
+	// determine sign
+	ch, _, err := r.ReadRune()
+	if err != nil {
+		return z, 0, err
+	}
+	neg := false
+	switch ch {
+	case '-':
+		neg = true
+	case '+': // nothing to do
+	default:
+		r.UnreadRune()
+	}
+
+	// determine mantissa
+	z.abs, base, err = z.abs.scan(r, base)
+	if err != nil {
+		return z, base, err
+	}
+	z.neg = len(z.abs) > 0 && neg // 0 has no sign
+
+	return z, base, nil
+}
+
+
+// Scan is a support routine for fmt.Scanner; it sets z to the value of
+// the scanned number. It accepts the formats 'b' (binary), 'o' (octal),
+// 'd' (decimal), 'x' (lowercase hexadecimal), and 'X' (uppercase hexadecimal).
+func (z *Int) Scan(s fmt.ScanState, ch int) os.Error {
+	base := 0
 	switch ch {
 	case 'b':
 		base = 2
@@ -394,32 +432,13 @@ func (x *Int) Scan(s fmt.ScanState, ch int) os.Error {
 	default:
 		return os.ErrorString("Int.Scan: invalid verb")
 	}
-
-	ch, _, err := s.ReadRune()
-	if err != nil {
-		return err
-	}
-	neg := false
-	switch ch {
-	case '-':
-		neg = true
-	case '+': // nothing to do
-	default:
-		s.UnreadRune()
-	}
-
-	x.abs, _, err = x.abs.scan(s, base)
-	if err != nil {
-		return err
-	}
-	x.neg = len(x.abs) > 0 && neg // 0 has no sign
-
-	return nil
+	_, _, err := z.scan(s, base)
+	return err
 }
 
 
-// Int64 returns the int64 representation of z.
-// If z cannot be represented in an int64, the result is undefined.
+// Int64 returns the int64 representation of x.
+// If x cannot be represented in an int64, the result is undefined.
 func (x *Int) Int64() int64 {
 	if len(x.abs) == 0 {
 		return 0
@@ -439,33 +458,19 @@ func (x *Int) Int64() int64 {
 // and returns z and a boolean indicating success. If SetString fails,
 // the value of z is undefined.
 //
-// If the base argument is 0, the string prefix determines the actual
-// conversion base. A prefix of ``0x'' or ``0X'' selects base 16; the
-// ``0'' prefix selects base 8, and a ``0b'' or ``0B'' prefix selects
-// base 2. Otherwise the selected base is 10.
+// The base argument must be 0 or a value from 2 through MaxBase. If the base
+// is 0, the string prefix determines the actual conversion base. A prefix of
+// ``0x'' or ``0X'' selects base 16; the ``0'' prefix selects base 8, and a
+// ``0b'' or ``0B'' prefix selects base 2. Otherwise the selected base is 10.
 //
 func (z *Int) SetString(s string, base int) (*Int, bool) {
-	neg := false
-	if len(s) > 0 {
-		switch s[0] {
-		case '-':
-			neg = true
-			fallthrough
-		case '+':
-			s = s[1:]
-		}
-	}
-
 	r := strings.NewReader(s)
-	abs, _, err := z.abs.scan(r, base)
+	_, _, err := z.scan(r, base)
 	if err != nil {
 		return z, false
 	}
 	_, _, err = r.ReadRune()
-
-	z.abs = abs
-	z.neg = len(abs) > 0 && neg // 0 has no sign
-	return z, err == os.EOF     // err == os.EOF => scan consumed all of s
+	return z, err == os.EOF // err == os.EOF => scan consumed all of s
 }
 
 
diff --git a/src/pkg/big/nat.go b/src/pkg/big/nat.go
index eb38750c168..313e22cbb31 100755
--- a/src/pkg/big/nat.go
+++ b/src/pkg/big/nat.go
@@ -608,8 +608,11 @@ func (x nat) bitLen() int {
 }
 
 
-func hexValue(ch int) int {
-	var d int
+// MaxBase is the largest number base accepted for string conversions.
+const MaxBase = 'z' - 'a' + 10 + 1 // = hexValue('z') + 1
+
+func hexValue(ch int) Word {
+	d := MaxBase + 1 // illegal base
 	switch {
 	case '0' <= ch && ch <= '9':
 		d = ch - '0'
@@ -617,86 +620,106 @@ func hexValue(ch int) int {
 		d = ch - 'a' + 10
 	case 'A' <= ch && ch <= 'Z':
 		d = ch - 'A' + 10
-	default:
-		return -1
 	}
-	return d
+	return Word(d)
 }
 
 
-// scan returns the natural number corresponding to the longest
-// possible prefix read from r representing a natural number in a
-// given conversion base, the actual conversion base used, and an
-// error, if any. The syntax of natural numbers follows the syntax of
+// scan sets z to the natural number corresponding to the longest possible prefix
+// read from r representing an unsigned integer in a given conversion base.
+// It returns z, the actual conversion base used, and an error, if any. In the
+// error case, the value of z is undefined. The syntax follows the syntax of
 // unsigned integer literals in Go.
 //
-// If the base argument is 0, the string prefix determines the actual
-// conversion base. A prefix of ``0x'' or ``0X'' selects base 16; the
-// ``0'' prefix selects base 8, and a ``0b'' or ``0B'' prefix selects
-// base 2. Otherwise the selected base is 10.
+// The base argument must be 0 or a value from 2 through MaxBase. If the base
+// is 0, the string prefix determines the actual conversion base. A prefix of
+// ``0x'' or ``0X'' selects base 16; the ``0'' prefix selects base 8, and a
+// ``0b'' or ``0B'' prefix selects base 2. Otherwise the selected base is 10.
 //
 func (z nat) scan(r io.RuneScanner, base int) (nat, int, os.Error) {
-	n := 0
+	// reject illegal bases
+	if base < 0 || base == 1 || MaxBase < base {
+		return z, 0, os.ErrorString("illegal number base")
+	}
+
+	// one char look-ahead
 	ch, _, err := r.ReadRune()
 	if err != nil {
 		return z, 0, err
 	}
+
 	// determine base if necessary
+	b := Word(base)
 	if base == 0 {
-		base = 10
+		b = 10
 		if ch == '0' {
-			n++
 			switch ch, _, err = r.ReadRune(); err {
 			case nil:
-				base = 8
+				b = 8
 				switch ch {
 				case 'x', 'X':
-					base = 16
+					b = 16
 				case 'b', 'B':
-					base = 2
+					b = 2
 				}
-				if base == 2 || base == 16 {
-					n--
+				if b == 2 || b == 16 {
 					if ch, _, err = r.ReadRune(); err != nil {
-						return z, 0, os.ErrorString("syntax error scanning binary or hexadecimal number")
+						return z, 0, err
 					}
 				}
 			case os.EOF:
 				return z, 10, nil
 			default:
-				return z, 0, err
+				return z, 10, err
 			}
 		}
 	}
 
-	// reject illegal bases
-	if base < 2 || 'z'-'a'+10 < base {
-		return z, 0, os.ErrorString("illegal number base")
-	}
-
 	// convert string
+	// - group as many digits d as possible together into a "super-digit" dd with "super-base" bb
+	// - only when bb does not fit into a word anymore, do a full number mulAddWW using bb and dd
 	z = z.make(0)
+	bb := Word(1)
+	dd := Word(0)
 	for {
 		d := hexValue(ch)
-		if 0 <= d && d < base {
-			z = z.mulAddWW(z, Word(base), Word(d))
-		} else {
-			r.UnreadRune()
-			if n > 0 {
-				break
-			}
-			return z, 0, os.ErrorString("syntax error scanning number")
+		if d >= b {
+			r.UnreadRune() // ch does not belong to number anymore
+			break
 		}
-		n++
+
+		if tmp := bb * b; tmp < bb {
+			// overflow
+			z = z.mulAddWW(z, bb, dd)
+			bb = b
+			dd = d
+		} else {
+			bb = tmp
+			dd = dd*b + d
+		}
+
 		if ch, _, err = r.ReadRune(); err != nil {
-			if err == os.EOF {
-				break
+			if err != os.EOF {
+				return z, int(b), err
 			}
-			return z, 0, err
+			break
 		}
 	}
 
-	return z.norm(), base, nil
+	switch {
+	case bb > 1:
+		// there was at least one mantissa digit
+		z = z.mulAddWW(z, bb, dd)
+	case base == 0 && b == 8:
+		// there was only the octal prefix 0 (possibly followed by digits > 7);
+		// return base 10, not 8
+		return z, 10, nil
+	case base != 0 || b != 8:
+		// there was neither a mantissa digit nor the octal prefix 0
+		return z, int(b), os.ErrorString("syntax error scanning number")
+	}
+
+	return z.norm(), int(b), nil
 }
 
 
diff --git a/src/pkg/big/nat_test.go b/src/pkg/big/nat_test.go
index 25947adda16..c358cc8cda6 100755
--- a/src/pkg/big/nat_test.go
+++ b/src/pkg/big/nat_test.go
@@ -5,6 +5,7 @@
 package big
 
 import (
+	"os"
 	"strings"
 	"testing"
 )
@@ -211,28 +212,58 @@ func TestString(t *testing.T) {
 
 var natScanTests = []struct {
 	s    string // string to be scanned
+	base int    // input base
 	x    nat    // expected nat
-	base int    // expected base
+	b    int    // expected base
 	ok   bool   // expected success
+	next int    // next character (or 0, if at EOF)
 }{
-	{s: ""},
-	{"0", nil, 10, true},
-	{"0 ", nil, 8, true},
+	// error: illegal base
+	{base: -1},
+	{base: 1},
+	{base: 37},
+
+	// error: no mantissa
+	{},
+	{s: "?"},
+	{base: 10},
+	{base: 36},
+	{s: "?", base: 10},
 	{s: "0x"},
-	{"08", nil, 8, true},
-	{"0b1", nat{1}, 2, true},
-	{"0b11000101", nat{0xc5}, 2, true},
-	{"03271", nat{03271}, 8, true},
-	{"10ab", nat{10}, 10, true},
-	{"1234567890", nat{1234567890}, 10, true},
-	{"0xdeadbeef", nat{0xdeadbeef}, 16, true},
-	{"0XDEADBEEF", nat{0xdeadbeef}, 16, true},
+	{s: "345", base: 2},
+
+	// no errors
+	{"0", 0, nil, 10, true, 0},
+	{"0", 10, nil, 10, true, 0},
+	{"0", 36, nil, 36, true, 0},
+	{"1", 0, nat{1}, 10, true, 0},
+	{"1", 10, nat{1}, 10, true, 0},
+	{"0 ", 0, nil, 10, true, ' '},
+	{"08", 0, nil, 10, true, '8'},
+	{"018", 0, nat{1}, 8, true, '8'},
+	{"0b1", 0, nat{1}, 2, true, 0},
+	{"0b11000101", 0, nat{0xc5}, 2, true, 0},
+	{"03271", 0, nat{03271}, 8, true, 0},
+	{"10ab", 0, nat{10}, 10, true, 'a'},
+	{"1234567890", 0, nat{1234567890}, 10, true, 0},
+	{"xyz", 36, nat{(33*36+34)*36 + 35}, 36, true, 0},
+	{"xyz?", 36, nat{(33*36+34)*36 + 35}, 36, true, '?'},
+	{"0x", 16, nil, 16, true, 'x'},
+	{"0xdeadbeef", 0, nat{0xdeadbeef}, 16, true, 0},
+	{"0XDEADBEEF", 0, nat{0xdeadbeef}, 16, true, 0},
+	{"0xfedcba9876543213fedcba9876543212fedcba9876543211fedcba9876543210",
+		0,
+		nat{0xfedcba9876543210, 0xfedcba9876543211, 0xfedcba9876543212, 0xfedcba9876543213},
+		16,
+		true,
+		0},
 }
 
 
-func TestScanBase0(t *testing.T) {
+func TestScanBase(t *testing.T) {
 	for _, a := range natScanTests {
-		x, b, err := nat(nil).scan(strings.NewReader(a.s), 0)
+		r := strings.NewReader(a.s)
+		x, b, err := nat(nil).scan(r, a.base)
 		if err == nil && !a.ok {
 			t.Errorf("scan%+v\n\texpected error", a)
 		}
@@ -245,9 +276,92 @@ func TestScanBase0(t *testing.T) {
 		if x.cmp(a.x) != 0 {
 			t.Errorf("scan%+v\n\tgot z = %v; want %v", a, x, a.x)
 		}
-		if b != a.base {
+		if b != a.b {
 			t.Errorf("scan%+v\n\tgot b = %d; want %d", a, b, a.base)
 		}
+		next, _, err := r.ReadRune()
+		if err == os.EOF {
+			next = 0
+			err = nil
+		}
+		if err == nil && next != a.next {
+			t.Errorf("scan%+v\n\tgot next = %q; want %q", a, next, a.next)
+		}
+	}
+}
+
+
+var pi = "3" +
+	"14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651" +
+	"32823066470938446095505822317253594081284811174502841027019385211055596446229489549303819644288109756659334461" +
+	"28475648233786783165271201909145648566923460348610454326648213393607260249141273724587006606315588174881520920" +
+	"96282925409171536436789259036001133053054882046652138414695194151160943305727036575959195309218611738193261179" +
+	"31051185480744623799627495673518857527248912279381830119491298336733624406566430860213949463952247371907021798" +
+	"60943702770539217176293176752384674818467669405132000568127145263560827785771342757789609173637178721468440901" +
+	"22495343014654958537105079227968925892354201995611212902196086403441815981362977477130996051870721134999999837" +
+	"29780499510597317328160963185950244594553469083026425223082533446850352619311881710100031378387528865875332083" +
+	"81420617177669147303598253490428755468731159562863882353787593751957781857780532171226806613001927876611195909" +
+	"21642019893809525720106548586327886593615338182796823030195203530185296899577362259941389124972177528347913151" +
+	"55748572424541506959508295331168617278558890750983817546374649393192550604009277016711390098488240128583616035" +
+	"63707660104710181942955596198946767837449448255379774726847104047534646208046684259069491293313677028989152104" +
+	"75216205696602405803815019351125338243003558764024749647326391419927260426992279678235478163600934172164121992" +
+	"45863150302861829745557067498385054945885869269956909272107975093029553211653449872027559602364806654991198818" +
+	"34797753566369807426542527862551818417574672890977772793800081647060016145249192173217214772350141441973568548" +
+	"16136115735255213347574184946843852332390739414333454776241686251898356948556209921922218427255025425688767179" +
+	"04946016534668049886272327917860857843838279679766814541009538837863609506800642251252051173929848960841284886" +
+	"26945604241965285022210661186306744278622039194945047123713786960956364371917287467764657573962413890865832645" +
+	"99581339047802759009946576407895126946839835259570982582262052248940772671947826848260147699090264013639443745" +
+	"53050682034962524517493996514314298091906592509372216964615157098583874105978859597729754989301617539284681382" +
+	"68683868942774155991855925245953959431049972524680845987273644695848653836736222626099124608051243884390451244" +
+	"13654976278079771569143599770012961608944169486855584840635342207222582848864815845602850601684273945226746767" +
+	"88952521385225499546667278239864565961163548862305774564980355936345681743241125150760694794510965960940252288" +
+	"79710893145669136867228748940560101503308617928680920874760917824938589009714909675985261365549781893129784821" +
+	"68299894872265880485756401427047755513237964145152374623436454285844479526586782105114135473573952311342716610" +
+	"21359695362314429524849371871101457654035902799344037420073105785390621983874478084784896833214457138687519435" +
+	"06430218453191048481005370614680674919278191197939952061419663428754440643745123718192179998391015919561814675" +
+	"14269123974894090718649423196156794520809514655022523160388193014209376213785595663893778708303906979207734672" +
+	"21825625996615014215030680384477345492026054146659252014974428507325186660021324340881907104863317346496514539" +
+	"05796268561005508106658796998163574736384052571459102897064140110971206280439039759515677157700420337869936007" +
+	"23055876317635942187312514712053292819182618612586732157919841484882916447060957527069572209175671167229109816" +
+	"90915280173506712748583222871835209353965725121083579151369882091444210067510334671103141267111369908658516398" +
+	"31501970165151168517143765761835155650884909989859982387345528331635507647918535893226185489632132933089857064" +
+	"20467525907091548141654985946163718027098199430992448895757128289059232332609729971208443357326548938239119325" +
+	"97463667305836041428138830320382490375898524374417029132765618093773444030707469211201913020330380197621101100" +
+	"44929321516084244485963766983895228684783123552658213144957685726243344189303968642624341077322697802807318915" +
+	"44110104468232527162010526522721116603966655730925471105578537634668206531098965269186205647693125705863566201" +
+	"85581007293606598764861179104533488503461136576867532494416680396265797877185560845529654126654085306143444318" +
+	"58676975145661406800700237877659134401712749470420562230538994561314071127000407854733269939081454664645880797" +
+	"27082668306343285878569830523580893306575740679545716377525420211495576158140025012622859413021647155097925923" +
+	"09907965473761255176567513575178296664547791745011299614890304639947132962107340437518957359614589019389713111" +
+	"79042978285647503203198691514028708085990480109412147221317947647772622414254854540332157185306142288137585043" +
+	"06332175182979866223717215916077166925474873898665494945011465406284336639379003976926567214638530673609657120" +
+	"91807638327166416274888800786925602902284721040317211860820419000422966171196377921337575114959501566049631862" +
+	"94726547364252308177036751590673502350728354056704038674351362222477158915049530984448933309634087807693259939" +
+	"78054193414473774418426312986080998886874132604721569516239658645730216315981931951673538129741677294786724229" +
+	"24654366800980676928238280689964004824354037014163149658979409243237896907069779422362508221688957383798623001" +
+	"59377647165122893578601588161755782973523344604281512627203734314653197777416031990665541876397929334419521541" +
+	"34189948544473456738316249934191318148092777710386387734317720754565453220777092120190516609628049092636019759" +
+	"88281613323166636528619326686336062735676303544776280350450777235547105859548702790814356240145171806246436267" +
+	"94561275318134078330336254232783944975382437205835311477119926063813346776879695970309833913077109870408591337"
+
+
+// Test case for BenchmarkScanPi.
+func TestScanPi(t *testing.T) {
+	var x nat
+	z, _, err := x.scan(strings.NewReader(pi), 10)
+	if err != nil {
+		t.Errorf("scanning pi: %s", err)
+	}
+	if s := z.decimalString(); s != pi {
+		t.Errorf("scanning pi: got %s", s)
+	}
+}
+
+
+func BenchmarkScanPi(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		var x nat
+		x.scan(strings.NewReader(pi), 10)
 	}
 }