diff --git a/enum.ml b/enum.ml
index 8a72da94..8c80f019 100644
--- a/enum.ml
+++ b/enum.ml
@@ -309,10 +309,106 @@ let merge enum =
             x
     in next
 
+(** {3 Mutable heap (taken from heap.ml to avoid dependencies)} *)
+module Heap = struct
+  type 'a t = {
+    mutable tree : 'a tree;
+    cmp : 'a -> 'a -> int;
+  } (** A splay tree heap with the given comparison function *)
+  and 'a tree =
+    | Empty
+    | Node of ('a tree * 'a * 'a tree)
+    (** A splay tree containing values of type 'a *)
+
+  let empty ~cmp = {
+    tree = Empty;
+    cmp;
+  }
+
+  let is_empty h =
+    match h.tree with
+    | Empty -> true
+    | Node _ -> false
+
+  (** Partition the tree into (elements <= pivot, elements > pivot) *)
+  let rec partition ~cmp pivot tree =
+    match tree with
+    | Empty -> Empty, Empty
+    | Node (a, x, b) ->
+      if cmp x pivot <= 0
+        then begin
+          match b with
+          | Empty -> (tree, Empty)
+          | Node (b1, y, b2) ->
+            if cmp y pivot <= 0
+              then
+                let small, big = partition ~cmp pivot b2 in
+                Node (Node (a, x, b1), y, small), big
+              else
+                let small, big = partition ~cmp pivot b1 in
+                Node (a, x, small), Node (big, y, b2)
+        end else begin
+          match a with
+          | Empty -> (Empty, tree)
+          | Node (a1, y, a2) ->
+            if cmp y pivot <= 0
+              then
+                let small, big = partition ~cmp pivot a2 in
+                Node (a1, y, small), Node (big, x, b)
+              else
+                let small, big = partition ~cmp pivot a1 in
+                small, Node (big, y, Node (a2, x, b))
+        end
+
+  (** Insert the element in the tree *)
+  let insert h x =
+    let small, big = partition ~cmp:h.cmp x h.tree in
+    let tree' = Node (small, x, big) in
+    h.tree <- tree'
+
+  (** Get minimum value and remove it from the tree *)
+  let pop h =
+    let rec delete_min tree = match tree with
+    | Empty -> raise Not_found
+    | Node (Empty, x, b) -> x, b
+    | Node (Node (Empty, x, b), y, c) ->
+      x, Node (b, y, c)  (* rebalance *)
+    | Node (Node (a, x, b), y, c) ->
+      let m, a' = delete_min a in
+      m, Node (a', x, Node (b, y, c))
+    in
+    let m, tree' = delete_min h.tree in
+    h.tree <- tree';
+    m
+end
+
 (** Assuming subsequences are sorted in increasing order, merge them
     into an increasing sequence *)
 let merge_sorted ?(cmp=compare) enum =
-  failwith "not implemented"
+  fun () ->
+    (* make a heap of (value, generator) *)
+    let cmp (v1,_) (v2,_) = cmp v1 v2 in
+    let heap = Heap.empty ~cmp in
+    (* add initial values *)
+    iter
+      (fun enum' ->
+        let gen = enum' () in
+        try
+          let x = gen () in
+          Heap.insert heap (x, gen)
+        with EOG -> ())
+      enum;
+    fun () ->
+      if Heap.is_empty heap then raise EOG
+      else begin
+        let x, gen = Heap.pop heap in
+        try
+          let y = gen () in
+          Heap.insert heap (y, gen);  (* insert next value *)
+          x
+        with EOG ->
+          x  (* gen is empty *)
+      end
 
 (** {3 Mutable double-linked list, similar to {! Deque.t} *)
 module MList = struct
diff --git a/enum.mli b/enum.mli
index db87070e..96832752 100644
--- a/enum.mli
+++ b/enum.mli
@@ -143,6 +143,15 @@ val merge : 'a t t -> 'a t
   (** Pick elements fairly in each sub-enum. The given enum
       must be finite (not its elements, though). *)
 
+(** {3 Mutable heap (taken from heap.ml to avoid dependencies)} *)
+module Heap : sig
+  type 'a t (** A heap containing values of type 'a *)
+  val empty : cmp:('a -> 'a -> int) -> 'a t
+  val insert : 'a t -> 'a -> unit
+  val is_empty : 'a t -> bool
+  val pop : 'a t -> 'a
+end
+
 val merge_sorted : ?cmp:('a -> 'a -> int) -> 'a t t -> 'a t
   (** Assuming subsequences are sorted in increasing order, merge them
       into an increasing sequence *)
diff --git a/tests/test_enum.ml b/tests/test_enum.ml
index 064359f1..463cf055 100644
--- a/tests/test_enum.ml
+++ b/tests/test_enum.ml
@@ -86,6 +86,12 @@ let test_big_rr () =
   OUnit.assert_equal [333;333;333] l';
   ()
 
+let test_merge_sorted () =
+  Enum.of_list [Enum.of_list [1;3;5]; Enum.of_list [0;1;1;3;4;6;10]; Enum.of_list [2;2;11]]
+    |> Enum.merge_sorted ?cmp:None
+    |> Enum.to_list
+    |> OUnit.assert_equal ~printer:Helpers.print_int_list [0;1;1;1;2;2;3;3;4;5;6;10;11]
+
 let test_interleave () =
   let e1 = Enum.of_list [1;3;5;7;9] in
   let e2 = Enum.of_list [2;4;6;8;10] in
@@ -117,6 +123,7 @@ let suite =
       "test_persistent" >:: test_persistent;
       "test_round_robin" >:: test_round_robin;
       "test_big_rr" >:: test_big_rr;
+      "test_merge_sorted" >:: test_merge_sorted;
       "test_interleave" >:: test_interleave;
       "test_intersperse" >:: test_intersperse;
       "test_product" >:: test_product;