diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index 72545a7509541b..a188160bb539ea 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -314,9 +314,11 @@ func (b *Reader) Buffered() int { return b.w - b.r }
 // ReadBytes or ReadString instead.
 // ReadSlice returns err != nil if and only if line does not end in delim.
 func (b *Reader) ReadSlice(delim byte) (line []byte, err error) {
+	s := 0 // search start index
 	for {
 		// Search buffer.
-		if i := bytes.IndexByte(b.buf[b.r:b.w], delim); i >= 0 {
+		if i := bytes.IndexByte(b.buf[b.r+s:b.w], delim); i >= 0 {
+			i += s
 			line = b.buf[b.r : b.r+i+1]
 			b.r += i + 1
 			break
@@ -338,6 +340,8 @@ func (b *Reader) ReadSlice(delim byte) (line []byte, err error) {
 			break
 		}
 
+		s = b.w - b.r // do not rescan area we scanned before
+
 		b.fill() // buffer is not full
 	}