diff --git a/src/libcore/benches/iter.rs b/src/libcore/benches/iter.rs
index 5b06229c21f..827c6354c60 100644
--- a/src/libcore/benches/iter.rs
+++ b/src/libcore/benches/iter.rs
@@ -146,3 +146,41 @@ fn bench_for_each_chain_ref_fold(b: &mut Bencher) {
         acc
     });
 }
+
+#[bench]
+fn bench_flat_map_sum(b: &mut Bencher) {
+    b.iter(|| -> i64 {
+        (0i64..1000).flat_map(|x| x..x+1000)
+            .map(black_box)
+            .sum()
+    });
+}
+
+#[bench]
+fn bench_flat_map_ref_sum(b: &mut Bencher) {
+    b.iter(|| -> i64 {
+        (0i64..1000).flat_map(|x| x..x+1000)
+            .map(black_box)
+            .by_ref()
+            .sum()
+    });
+}
+
+#[bench]
+fn bench_flat_map_chain_sum(b: &mut Bencher) {
+    b.iter(|| -> i64 {
+        (0i64..1000000).flat_map(|x| once(x).chain(once(x)))
+            .map(black_box)
+            .sum()
+    });
+}
+
+#[bench]
+fn bench_flat_map_chain_ref_sum(b: &mut Bencher) {
+    b.iter(|| -> i64 {
+        (0i64..1000000).flat_map(|x| once(x).chain(once(x)))
+            .map(black_box)
+            .by_ref()
+            .sum()
+    });
+}
diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index ebedfe1d743..a596ffd6ae8 100644
--- a/src/libcore/iter/mod.rs
+++ b/src/libcore/iter/mod.rs
@@ -1902,6 +1902,16 @@ impl<I: Iterator, U: IntoIterator, F> Iterator for FlatMap<I, U, F>
             _ => (lo, None)
         }
     }
+
+    #[inline]
+    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
+        where Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.frontiter.into_iter()
+            .chain(self.iter.map(self.f).map(U::into_iter))
+            .chain(self.backiter)
+            .fold(init, |acc, iter| iter.fold(acc, &mut fold))
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]