diff --git a/src/fmt/doc.go b/src/fmt/doc.go
index b7eaedc11e..00dd8d01cd 100644
--- a/src/fmt/doc.go
+++ b/src/fmt/doc.go
@@ -63,16 +63,20 @@
 		%9.2f  width 9, precision 2
 		%9.f   width 9, precision 0
 
-	Width and precision are measured in units of Unicode code points.
-	(This differs from C's printf where the units are numbers
-	of bytes.) Either or both of the flags may be replaced with the
-	character '*', causing their values to be obtained from the next
-	operand, which must be of type int.
+	Width and precision are measured in units of Unicode code points,
+	that is, runes. (This differs from C's printf where the
+	units are always measured in bytes.) Either or both of the flags
+	may be replaced with the character '*', causing their values to be
+	obtained from the next operand, which must be of type int.
 
-	For most values, width is the minimum number of characters to output,
+	For most values, width is the minimum number of runes to output,
 	padding the formatted form with spaces if necessary.
-	For strings, precision is the maximum number of characters to output,
-	truncating if necessary.
+
+	For strings, byte slices and byte arrays, however, precision
+	limits the length of the input to be formatted (not the size of
+	the output), truncating if necessary. Normally it is measured in
+	runes, but for these types when formatted with the %x or %X format
+	it is measured in bytes.
 
 	For floating-point values, width sets the minimum width of the field and
 	precision sets the number of places after the decimal, if appropriate,
diff --git a/src/fmt/fmt_test.go b/src/fmt/fmt_test.go
index cca0a495ff..4586fcf933 100644
--- a/src/fmt/fmt_test.go
+++ b/src/fmt/fmt_test.go
@@ -194,8 +194,15 @@ var fmtTests = []struct {
 	{"%.5s", "日本語日本語", "日本語日本"},
 	{"%.5s", []byte("日本語日本語"), "日本語日本"},
 	{"%.5q", "abcdefghijklmnopqrstuvwxyz", `"abcde"`},
+	{"%.5x", "abcdefghijklmnopqrstuvwxyz", `6162636465`},
+	{"%.5q", []byte("abcdefghijklmnopqrstuvwxyz"), `"abcde"`},
+	{"%.5x", []byte("abcdefghijklmnopqrstuvwxyz"), `6162636465`},
 	{"%.3q", "日本語日本語", `"日本語"`},
 	{"%.3q", []byte("日本語日本語"), `"日本語"`},
+	{"%.1q", "日本語", `"日"`},
+	{"%.1q", []byte("日本語"), `"日"`},
+	{"%.1x", "日本語", `e6`},
+	{"%.1X", []byte("日本語"), `E6`},
 	{"%10.1q", "日本語日本語", `       "日"`},
 	{"%3c", '⌘', "  ⌘"},
 	{"%5q", '\u2026', `  '…'`},
diff --git a/src/fmt/format.go b/src/fmt/format.go
index 255167c8f5..a92f3c2f86 100644
--- a/src/fmt/format.go
+++ b/src/fmt/format.go
@@ -340,11 +340,17 @@ func (f *fmt) fmt_sbx(s string, b []byte, digits string) {
 
 // fmt_sx formats a string as a hexadecimal encoding of its bytes.
 func (f *fmt) fmt_sx(s, digits string) {
+	if f.precPresent && f.prec < len(s) {
+		s = s[:f.prec]
+	}
 	f.fmt_sbx(s, nil, digits)
 }
 
 // fmt_bx formats a byte slice as a hexadecimal encoding of its bytes.
 func (f *fmt) fmt_bx(b []byte, digits string) {
+	if f.precPresent && f.prec < len(b) {
+		b = b[:f.prec]
+	}
 	f.fmt_sbx("", b, digits)
 }