diff --git a/src/pkg/archive/tar/Makefile b/src/pkg/archive/tar/Makefile
index 579ed4c351..2689b30f6a 100644
--- a/src/pkg/archive/tar/Makefile
+++ b/src/pkg/archive/tar/Makefile
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
+
 # DO NOT EDIT.  Automatically generated by gobuild.
 # gobuild -m >Makefile
 
@@ -20,7 +21,7 @@ test: packages
 
 coverage: packages
 	gotest
-	6cov -g `pwd` | grep -v '_test\.go:'
+	6cov -g $$(pwd) | grep -v '_test\.go:'
 
 %.$O: %.go
 	$(GC) -I_obj $*.go
@@ -32,16 +33,24 @@ coverage: packages
 	$(AS) $*.s
 
 O1=\
+	common.$O\
+
+O2=\
 	untar.$O\
+	writer.$O\
 
 
-phases: a1
+phases: a1 a2
 _obj$D/tar.a: phases
 
 a1: $(O1)
-	$(AR) grc _obj$D/tar.a untar.$O
+	$(AR) grc _obj$D/tar.a common.$O
 	rm -f $(O1)
 
+a2: $(O2)
+	$(AR) grc _obj$D/tar.a untar.$O writer.$O
+	rm -f $(O2)
+
 
 newpkg: clean
 	mkdir -p _obj$D
@@ -49,6 +58,7 @@ newpkg: clean
 
 $(O1): newpkg
 $(O2): a1
+$(O3): a2
 
 nuke: clean
 	rm -f $(GOROOT)/pkg/$(GOOS)_$(GOARCH)$D/tar.a
diff --git a/src/pkg/archive/tar/common.go b/src/pkg/archive/tar/common.go
new file mode 100644
index 0000000000..182d465278
--- /dev/null
+++ b/src/pkg/archive/tar/common.go
@@ -0,0 +1,74 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The tar package implements access to tar archives.
+// It aims to cover most of the variations, including those produced
+// by GNU and BSD tars.
+//
+// References:
+//   http://www.freebsd.org/cgi/man.cgi?query=tar&sektion=5
+//   http://www.gnu.org/software/tar/manual/html_node/Standard.html
+package tar
+
+const (
+	blockSize = 512;
+
+	// Types
+	TypeReg = '0';
+	TypeRegA = '\x00';
+	TypeLink = '1';
+	TypeSymlink = '2';
+	TypeChar = '3';
+	TypeBlock = '4';
+	TypeDir = '5';
+	TypeFifo = '6';
+	TypeCont = '7';
+	TypeXHeader = 'x';
+	TypeXGlobalHeader = 'g';
+)
+
+// A Header represents a single header in a tar archive.
+// Some fields may not be populated.
+type Header struct {
+	Name string;
+	Mode int64;
+	Uid int64;
+	Gid int64;
+	Size int64;
+	Mtime int64;
+	Typeflag byte;
+	Linkname string;
+	Uname string;
+	Gname string;
+	Devmajor int64;
+	Devminor int64;
+	Atime int64;
+	Ctime int64;
+}
+
+var zeroBlock = make([]byte, blockSize);
+
+// POSIX specifies a sum of the unsigned byte values, but the Sun tar uses signed byte values.
+// We compute and return both.
+func checksum(header []byte) (unsigned int64, signed int64) {
+	for i := 0; i < len(header); i++ {
+		if i == 148 {
+			// The chksum field (header[148:156]) is special: it should be treated as space bytes.
+			unsigned += ' ' * 8;
+			signed += ' ' * 8;
+			i += 7;
+			continue
+		}
+		unsigned += int64(header[i]);
+		signed += int64(int8(header[i]));
+	}
+	return
+}
+
+type slicer []byte
+func (sp *slicer) next(n int) (b []byte) {
+	s := *sp;
+	b, *sp = s[0:n], s[n:len(s)];
+	return
+}
diff --git a/src/pkg/archive/tar/testdata/writer.tar b/src/pkg/archive/tar/testdata/writer.tar
new file mode 100644
index 0000000000..0358f91b98
Binary files /dev/null and b/src/pkg/archive/tar/testdata/writer.tar differ
diff --git a/src/pkg/archive/tar/untar.go b/src/pkg/archive/tar/untar.go
index 3ebfc5e562..87382d4f5c 100644
--- a/src/pkg/archive/tar/untar.go
+++ b/src/pkg/archive/tar/untar.go
@@ -2,20 +2,14 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// The tar package implements access to tar archives.
-// It aims to cover most of the variations, including those produced
-// by GNU and BSD tars.
-//
-// References:
-//   http://www.freebsd.org/cgi/man.cgi?query=tar&sektion=5
-//   http://www.gnu.org/software/tar/manual/html_node/Standard.html
 package tar
 
 // TODO(dsymonds):
 //   - pax extensions
+//   - rename this file to reader.go
 
 import (
-	"bufio";
+	"archive/tar";
 	"bytes";
 	"io";
 	"os";
@@ -51,25 +45,6 @@ type Reader struct {
 	pad int64;	// amount of padding (ignored) after current file entry
 }
 
-// A Header represents a single header in a tar archive.
-// Only some fields may be populated.
-type Header struct {
-	Name string;
-	Mode int64;
-	Uid int64;
-	Gid int64;
-	Size int64;
-	Mtime int64;
-	Typeflag byte;
-	Linkname string;
-	Uname string;
-	Gname string;
-	Devmajor int64;
-	Devminor int64;
-	Atime int64;
-	Ctime int64;
-}
-
 func (tr *Reader) skipUnread()
 func (tr *Reader) readHeader() *Header
 
@@ -90,25 +65,6 @@ func (tr *Reader) Next() (*Header, os.Error) {
 	return hdr, tr.err
 }
 
-const (
-	blockSize = 512;
-
-	// Types
-	TypeReg = '0';
-	TypeRegA = '\x00';
-	TypeLink = '1';
-	TypeSymlink = '2';
-	TypeChar = '3';
-	TypeBlock = '4';
-	TypeDir = '5';
-	TypeFifo = '6';
-	TypeCont = '7';
-	TypeXHeader = 'x';
-	TypeXGlobalHeader = 'g';
-)
-
-var zeroBlock = make([]byte, blockSize);
-
 // Parse bytes as a NUL-terminated C-style string.
 // If a NUL byte is not found then the whole slice is returned as a string.
 func cString(b []byte) string {
@@ -153,36 +109,15 @@ func (tr *Reader) skipUnread() {
 }
 
 func (tr *Reader) verifyChecksum(header []byte) bool {
-	given := tr.octal(header[148:156]);
 	if tr.err != nil {
 		return false
 	}
 
-	// POSIX specifies a sum of the unsigned byte values,
-	// but the Sun tar uses signed byte values.  :-(
-	var unsigned, signed int64;
-	for i := 0; i < len(header); i++ {
-		if i == 148 {
-			// The chksum field is special: it should be treated as space bytes.
-			unsigned += ' ' * 8;
-			signed += ' ' * 8;
-			i += 7;
-			continue
-		}
-		unsigned += int64(header[i]);
-		signed += int64(int8(header[i]));
-	}
-
+	given := tr.octal(header[148:156]);
+	unsigned, signed := checksum(header);
 	return given == unsigned || given == signed
 }
 
-type slicer []byte
-func (sp *slicer) next(n int) (b []byte) {
-	s := *sp;
-	b, *sp = s[0:n], s[n:len(s)];
-	return
-}
-
 func (tr *Reader) readHeader() *Header {
 	header := make([]byte, blockSize);
 	var n int;
diff --git a/src/pkg/archive/tar/untar_test.go b/src/pkg/archive/tar/untar_test.go
index 9a42c9c926..a3a02978a7 100644
--- a/src/pkg/archive/tar/untar_test.go
+++ b/src/pkg/archive/tar/untar_test.go
@@ -102,9 +102,9 @@ var untarTests = []*untarTest{
 			},
 		},
 	},
-};
+}
 
-func TestAll(t *testing.T) {
+func TestReader(t *testing.T) {
 testLoop:
 	for i, test := range untarTests {
 		f, err := os.Open(test.file, os.O_RDONLY, 0444);
diff --git a/src/pkg/archive/tar/writer.go b/src/pkg/archive/tar/writer.go
new file mode 100644
index 0000000000..57e9a4607c
--- /dev/null
+++ b/src/pkg/archive/tar/writer.go
@@ -0,0 +1,180 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tar
+
+// TODO(dsymonds):
+// - catch more errors (no first header, write after close, etc.)
+
+import (
+	"archive/tar";
+	"bytes";
+	"io";
+	"os";
+	"strconv";
+	"strings";
+)
+
+var (
+	ErrWriteTooLong os.Error = os.ErrorString("write too long");
+	// TODO(dsymonds): remove ErrIntFieldTooBig after we implement binary extension.
+	ErrIntFieldTooBig os.Error = os.ErrorString("an integer header field was too big");
+)
+
+// A Writer provides sequential writing of a tar archive in POSIX.1 format.
+// A tar archive consists of a sequence of files.
+// Call WriteHeader to begin a new file, and then call Write to supply that file's data,
+// writing at most hdr.Size bytes in total.
+//
+// Example:
+// 	tw := NewTarWriter(w);
+//	hdr := new(Header);
+//	hdr.Size = length of data in bytes;
+//	// populate other hdr fields as desired
+//	if err := tw.WriteHeader(hdr); err != nil {
+//		// handle error
+//	}
+//	io.Copy(data, tw);
+//	tw.Close();
+type Writer struct {
+	w io.Writer;
+	err os.Error;
+	nb int64;	// number of unwritten bytes for current file entry
+	pad int64;	// amount of padding to write after current file entry
+	closed bool;
+}
+
+// NewWriter creates a new Writer writing to w.
+func NewWriter(w io.Writer) *Writer {
+	return &Writer{ w: w }
+}
+
+// Flush finishes writing the current file (optional).
+func (tw *Writer) Flush() os.Error {
+	n := tw.nb + tw.pad;
+	for n > 0 && tw.err == nil {
+		nr := n;
+		if nr > blockSize {
+			nr = blockSize;
+		}
+		var nw int;
+		nw, tw.err = tw.w.Write(zeroBlock[0:nr]);
+		n -= int64(nw);
+	}
+	tw.nb = 0;
+	tw.pad = 0;
+	return tw.err
+}
+
+// Write s into b, terminating it with a NUL if there is room.
+func (tw *Writer) cString(b []byte, s string) {
+	if len(s) > len(b) {
+		if tw.err == nil {
+			tw.err = ErrIntFieldTooBig;
+		}
+		return
+	}
+	for i, ch := range strings.Bytes(s) {
+		b[i] = ch;
+	}
+	if len(s) < len(b) {
+		b[len(s)] = 0;
+	}
+}
+
+// Encode x as an octal ASCII string and write it into b with leading zeros.
+func (tw *Writer) octal(b []byte, x int64) {
+	s := strconv.Itob64(x, 8);
+	// leading zeros, but leave room for a NUL.
+	for len(s) + 1 < len(b) {
+		s = "0" + s;
+	}
+	tw.cString(b, s);
+}
+
+// WriteHeader writes hdr and prepares to accept the file's contents.
+// WriteHeader calls Flush if it is not the first header.
+func (tw *Writer) WriteHeader(hdr *Header) os.Error {
+	if tw.err == nil {
+		tw.Flush();
+	}
+	if tw.err != nil {
+		return tw.err
+	}
+
+	tw.nb = int64(hdr.Size);
+	tw.pad = -tw.nb & (blockSize - 1);  // blockSize is a power of two
+
+	header := make([]byte, blockSize);
+	s := slicer(header);
+
+	// TODO(dsymonds): handle names longer than 100 chars
+	nr := bytes.Copy(s.next(100), strings.Bytes(hdr.Name));
+
+	tw.octal(s.next(8), hdr.Mode);
+	tw.octal(s.next(8), hdr.Uid);
+	tw.octal(s.next(8), hdr.Gid);
+	tw.octal(s.next(12), hdr.Size);
+	tw.octal(s.next(12), hdr.Mtime);
+	s.next(8);  // chksum
+	s.next(1)[0] = hdr.Typeflag;
+	s.next(100);  // linkname
+	bytes.Copy(s.next(8), strings.Bytes("ustar\x0000"));
+	tw.cString(s.next(32), hdr.Uname);
+	tw.cString(s.next(32), hdr.Gname);
+	tw.octal(s.next(8), hdr.Devmajor);
+	tw.octal(s.next(8), hdr.Devminor);
+
+	// The chksum field is terminated by a NUL and a space.
+	// This is different from the other octal fields.
+	chksum, _ := checksum(header);
+	tw.octal(header[148:155], chksum);
+	header[155] = ' ';
+
+	if tw.err != nil {
+		// problem with header; probably integer too big for a field.
+		return tw.err
+	}
+
+	var n int;
+	n, tw.err = tw.w.Write(header);
+
+	return tw.err
+}
+
+// Write writes to the current entry in the tar archive.
+// Write returns the error ErrWriteTooLong if more than
+// hdr.Size bytes are written after WriteHeader.
+func (tw *Writer) Write(b []uint8) (n int, err os.Error) {
+	overwrite := false;
+	if int64(len(b)) > tw.nb {
+		b = b[0:tw.nb];
+		overwrite = true;
+	}
+	n, err = tw.w.Write(b);
+	tw.nb -= int64(n);
+	if err == nil && overwrite {
+		err = ErrWriteTooLong;
+	}
+	tw.err = err;
+	return
+}
+
+func (tw *Writer) Close() os.Error {
+	if tw.err != nil || tw.closed {
+		return tw.err
+	}
+	tw.Flush();
+	tw.closed = true;
+
+	// trailer: two zero blocks
+	for i := 0; i < 2; i++ {
+		var n int;
+		n, tw.err = tw.w.Write(zeroBlock);
+		if tw.err != nil {
+			break
+		}
+	}
+	return tw.err
+}
diff --git a/src/pkg/archive/tar/writer_test.go b/src/pkg/archive/tar/writer_test.go
new file mode 100644
index 0000000000..202530a0f0
--- /dev/null
+++ b/src/pkg/archive/tar/writer_test.go
@@ -0,0 +1,122 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tar
+
+import (
+	"archive/tar";
+	"bytes";
+	"fmt";
+	"io";
+	"os";
+	"reflect";
+	"strings";
+	"testing";
+)
+
+type writerTestEntry struct {
+	header *Header;
+	contents string;
+}
+
+type writerTest struct {
+	file string;  // filename of expected output
+	entries []*writerTestEntry;
+}
+
+var writerTests = []*writerTest{
+	&writerTest{
+		file: "testdata/writer.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name: "small.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 5,
+					Mtime: 1246508266,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				contents: `Kilts`,
+			},
+			&writerTestEntry{
+				header: &Header{
+					Name: "small2.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 11,
+					Mtime: 1245217492,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				contents: "Google.com\n",
+			},
+		}
+	},
+}
+
+// Render byte array in a two-character hexadecimal string, spaced for easy visual inspection.
+func bytestr(b []byte) string {
+	s := fmt.Sprintf("(%d bytes)\n", len(b));
+	const rowLen = 32;
+	for i, ch := range b {
+		if i % rowLen == 0 {
+			// start of line: hex offset
+			s += fmt.Sprintf("%04x", i);
+		}
+		switch {
+		case '0' <= ch && ch <= '9', 'A' <= ch && ch <= 'Z', 'a' <= ch && ch <= 'z':
+			s += fmt.Sprintf("  %c", ch);
+		default:
+			s += fmt.Sprintf(" %02x", ch);
+		}
+		if (i + 1) % rowLen == 0 {
+			// end of line
+			s += "\n";
+		} else if (i + 1) % (rowLen / 2) == 0 {
+			// extra space
+			s += " ";
+		}
+	}
+	if s[len(s)-1] != '\n' {
+		s += "\n"
+	}
+	return s
+}
+
+func TestWriter(t *testing.T) {
+testLoop:
+	for i, test := range writerTests {
+		expected, err := io.ReadFile(test.file);
+		if err != nil {
+			t.Errorf("test %d: Unexpected error: %v", i, err);
+			continue
+		}
+
+		buf := new(bytes.Buffer);
+		tw := NewWriter(buf);
+		for j, entry := range test.entries {
+			if err := tw.WriteHeader(entry.header); err != nil {
+				t.Errorf("test %d, entry %d: Failed writing header: %v", i, j, err);
+				continue testLoop
+			}
+			if n, err := io.WriteString(tw, entry.contents); err != nil {
+				t.Errorf("test %d, entry %d: Failed writing contents: %v", i, j, err);
+				continue testLoop
+			}
+		}
+		tw.Close();
+
+		actual := buf.Data();
+		if !bytes.Equal(expected, actual) {
+			t.Errorf("test %d: Incorrect result:\n%v\nwant:\n%v",
+				 i, bytestr(actual), bytestr(expected));
+		}
+	}
+}