diff --git a/src/strconv/atoi.go b/src/strconv/atoi.go
index ecbc9f4..31774d0 100644
--- a/src/strconv/atoi.go
+++ b/src/strconv/atoi.go
@@ -71,18 +71,22 @@
 
 	case base == 0:
 		// Look for octal, hex prefix.
-		switch {
-		case s[0] == '0' && len(s) >= 3 && lower(s[1]) == 'x':
-			if len(s) < 3 {
-				return 0, syntaxError(fnParseUint, s0)
+		base = 10
+		if s[0] == '0' {
+			switch {
+			case len(s) >= 3 && lower(s[1]) == 'b':
+				base = 2
+				s = s[2:]
+			case len(s) >= 3 && lower(s[1]) == 'o':
+				base = 8
+				s = s[2:]
+			case len(s) >= 3 && lower(s[1]) == 'x':
+				base = 16
+				s = s[2:]
+			default:
+				base = 8
+				s = s[1:]
 			}
-			base = 16
-			s = s[2:]
-		case s[0] == '0':
-			base = 8
-			s = s[1:]
-		default:
-			base = 10
 		}
 
 	default:
@@ -149,13 +153,14 @@
 // bit size (0 to 64) and returns the corresponding value i.
 //
 // If base == 0, the base is implied by the string's prefix:
-// base 16 for "0x", base 8 for "0", and base 10 otherwise.
-// For bases 1, below 0 or above 36 an error is returned.
+// base 2 for "0b", base 8 for "0" or "0o", base 16 for "0x",
+// and base 10 otherwise.
+// If base is below 0, is 1, or is above 36, an error is returned.
 //
 // The bitSize argument specifies the integer type
 // that the result must fit into. Bit sizes 0, 8, 16, 32, and 64
 // correspond to int, int8, int16, int32, and int64.
-// For a bitSize below 0 or above 64 an error is returned.
+// If bitSize is below 0 or above 64, an error is returned.
 //
 // The errors that ParseInt returns have concrete type *NumError
 // and include err.Num = s. If s is empty or contains invalid
diff --git a/src/strconv/atoi_test.go b/src/strconv/atoi_test.go
index ec0542ab..8b0576b 100644
--- a/src/strconv/atoi_test.go
+++ b/src/strconv/atoi_test.go
@@ -65,42 +65,69 @@
 	{"01777777777777777777778", 0, 0, ErrSyntax},
 	{"02000000000000000000000", 0, 1<<64 - 1, ErrRange},
 	{"0200000000000000000000", 0, 1 << 61, nil},
+	{"0b", 0, 0, ErrSyntax},
+	{"0B", 0, 0, ErrSyntax},
+	{"0b101", 0, 5, nil},
+	{"0B101", 0, 5, nil},
+	{"0o", 0, 0, ErrSyntax},
+	{"0O", 0, 0, ErrSyntax},
+	{"0o377", 0, 255, nil},
+	{"0O377", 0, 255, nil},
 
 	// underscores allowed with base == 0 only
-	{"1_2_3_4_5", 0, 12345, nil},
+	{"1_2_3_4_5", 0, 12345, nil}, // base 0 => 10
 	{"_12345", 0, 0, ErrSyntax},
 	{"1__2345", 0, 0, ErrSyntax},
 	{"12345_", 0, 0, ErrSyntax},
 
-	{"1_2_3_4_5", 10, 0, ErrSyntax},
+	{"1_2_3_4_5", 10, 0, ErrSyntax}, // base 10
 	{"_12345", 10, 0, ErrSyntax},
 	{"1__2345", 10, 0, ErrSyntax},
 	{"12345_", 10, 0, ErrSyntax},
 
-	{"0x_1_2_3_4_5", 0, 0x12345, nil},
+	{"0x_1_2_3_4_5", 0, 0x12345, nil}, // base 0 => 16
 	{"_0x12345", 0, 0, ErrSyntax},
 	{"0x__12345", 0, 0, ErrSyntax},
 	{"0x1__2345", 0, 0, ErrSyntax},
 	{"0x1234__5", 0, 0, ErrSyntax},
 	{"0x12345_", 0, 0, ErrSyntax},
 
-	{"1_2_3_4_5", 16, 0, ErrSyntax},
+	{"1_2_3_4_5", 16, 0, ErrSyntax}, // base 16
 	{"_12345", 16, 0, ErrSyntax},
 	{"1__2345", 16, 0, ErrSyntax},
 	{"1234__5", 16, 0, ErrSyntax},
 	{"12345_", 16, 0, ErrSyntax},
 
-	{"0_1_2_3_4_5", 0, 012345, nil},
+	{"0_1_2_3_4_5", 0, 012345, nil}, // base 0 => 8 (0377)
 	{"_012345", 0, 0, ErrSyntax},
 	{"0__12345", 0, 0, ErrSyntax},
 	{"01234__5", 0, 0, ErrSyntax},
 	{"012345_", 0, 0, ErrSyntax},
 
-	{"0_1_2_3_4_5", 8, 0, ErrSyntax},
+	{"0o_1_2_3_4_5", 0, 012345, nil}, // base 0 => 8 (0o377)
+	{"_0o12345", 0, 0, ErrSyntax},
+	{"0o__12345", 0, 0, ErrSyntax},
+	{"0o1234__5", 0, 0, ErrSyntax},
+	{"0o12345_", 0, 0, ErrSyntax},
+
+	{"0_1_2_3_4_5", 8, 0, ErrSyntax}, // base 8
 	{"_012345", 8, 0, ErrSyntax},
 	{"0__12345", 8, 0, ErrSyntax},
 	{"01234__5", 8, 0, ErrSyntax},
 	{"012345_", 8, 0, ErrSyntax},
+
+	{"0b_1_0_1", 0, 5, nil}, // base 0 => 2 (0b101)
+	{"_0b101", 0, 0, ErrSyntax},
+	{"0b__101", 0, 0, ErrSyntax},
+	{"0b1__01", 0, 0, ErrSyntax},
+	{"0b10__1", 0, 0, ErrSyntax},
+	{"0b101_", 0, 0, ErrSyntax},
+
+	{"1_0_1", 2, 0, ErrSyntax}, // base 2
+	{"_101", 2, 0, ErrSyntax},
+	{"1_01", 2, 0, ErrSyntax},
+	{"10_1", 2, 0, ErrSyntax},
+	{"101_", 2, 0, ErrSyntax},
 }
 
 type parseInt64Test struct {