diff --git a/src/pkg/fmt/doc.go b/src/pkg/fmt/doc.go
index 06dc730089..a026a5e197 100644
--- a/src/pkg/fmt/doc.go
+++ b/src/pkg/fmt/doc.go
@@ -58,7 +58,7 @@
 			0X for hex (%#X); suppress 0x for %p (%#p);
 			print a raw (backquoted) string if possible for %q (%#q)
 		' '	(space) leave a space for elided sign in numbers (% d);
-			put spaces between bytes printing strings or slices in hex (% x)
+			put spaces between bytes printing strings or slices in hex (% x, % X)
 		0	pad with leading zeros rather than spaces
 
 	For each Printf-like function, there is also a Print function
diff --git a/src/pkg/fmt/fmt_test.go b/src/pkg/fmt/fmt_test.go
index 2c09e0713b..fbc2536ee1 100644
--- a/src/pkg/fmt/fmt_test.go
+++ b/src/pkg/fmt/fmt_test.go
@@ -121,7 +121,8 @@ var fmttests = []fmtTest{
 	// basic bytes
 	{"%s", []byte("abc"), "abc"},
 	{"%x", []byte("abc"), "616263"},
-	{"% x", []byte("abc"), "61 62 63"},
+	{"% x", []byte("abc\xff"), "61 62 63 ff"},
+	{"% X", []byte("abc\xff"), "61 62 63 FF"},
 	{"%x", []byte("xyz"), "78797a"},
 	{"%X", []byte("xyz"), "78797A"},
 	{"%q", []byte("abc"), `"abc"`},
diff --git a/src/pkg/fmt/format.go b/src/pkg/fmt/format.go
index 3ec1cf1394..010280bf85 100644
--- a/src/pkg/fmt/format.go
+++ b/src/pkg/fmt/format.go
@@ -255,6 +255,9 @@ func (f *fmt) fmt_sx(s string) {
 func (f *fmt) fmt_sX(s string) {
 	t := ""
 	for i := 0; i < len(s); i++ {
+		if i > 0 && f.space {
+			t += " "
+		}
 		v := s[i]
 		t += string(udigits[v>>4])
 		t += string(udigits[v&0xF])