diff --git a/graph.ml b/graph.ml
index 3346463d..c721fbee 100644
--- a/graph.ml
+++ b/graph.ml
@@ -92,7 +92,7 @@ let prev t v =
   Sequence.of_list (PHashtbl.find t v).n_prev
 
 let between t v1 v2 =
-  let edges = Sequence.of_list (PHashtbl.find t v1).n_prev in
+  let edges = Sequence.of_list (PHashtbl.find t v1).n_next in
   let edges = Sequence.filter (fun (e, v2') -> (PHashtbl.get_eq t) v2 v2') edges in
   Sequence.map fst edges
 
diff --git a/tests/helpers.ml b/tests/helpers.ml
new file mode 100644
index 00000000..28f71b3a
--- /dev/null
+++ b/tests/helpers.ml
@@ -0,0 +1,9 @@
+
+(** Some helpers for tests *)
+
+let print_int_list l =
+  let b = Buffer.create 20 in
+  Format.bprintf b "@[<h>[%a]@]"
+    (Sequence.pp_seq ~sep:", " Format.pp_print_int)
+    (Sequence.of_list l);
+  Buffer.contents b
diff --git a/tests/test_graph.ml b/tests/test_graph.ml
new file mode 100644
index 00000000..1a07898d
--- /dev/null
+++ b/tests/test_graph.ml
@@ -0,0 +1,50 @@
+
+(** Tests on graphs *)
+
+open OUnit
+open Helpers
+
+(* build a graph from a list of pairs of ints *)
+let mk_graph l =
+  let g = Graph.empty 5 in
+  Graph.add_seq g
+    (Sequence.map (fun (x,y) -> x,1,y)
+      (Sequence.of_list l));
+  g
+
+let test_copy () =
+  let g = mk_graph [0,1; 1,2; 2,3; 3,0] in
+  let g' = Graph.copy g in
+  Graph.add g 1 1 3;
+  Graph.add g 1 2 3;
+  OUnit.assert_equal ~printer:print_int_list
+    [1;2] (List.sort compare (Sequence.to_list (Graph.between g 1 3)));
+  OUnit.assert_bool "copy" (Sequence.is_empty (Graph.between g' 1 3));
+  ()
+
+let test_roots () =
+  let g = mk_graph [0,1; 1,2; 2,3; 4,1; 5,1; 6,5; 3,5] in
+  let roots = Sequence.to_list (Graph.roots g) in
+  OUnit.assert_equal (List.sort compare roots) [0;4;6]
+
+let test_leaves () =
+  let g = mk_graph [0,1; 1,2; 2,3; 4,1; 6,5; 3,5; 3,7] in
+  let leaves = Sequence.to_list (Graph.leaves g) in
+  OUnit.assert_equal (List.sort compare leaves) [5;7]
+
+let test_dfs () =
+  let g = mk_graph [0,1; 1,2; 2,3; 3,0] in
+
+  ()
+
+let test_dijkstra () =
+  ()
+
+let suite =
+  "test_graph" >:::
+    [ "test_copy" >:: test_copy;
+      "test_leaves" >:: test_leaves;
+      "test_roots" >:: test_roots;
+      "test_dfs" >:: test_dfs;
+      "test_dijkstra" >:: test_dijkstra;
+    ]
diff --git a/tests/test_heap.ml b/tests/test_heap.ml
index 3b10cd94..3b24fcdb 100644
--- a/tests/test_heap.ml
+++ b/tests/test_heap.ml
@@ -1,13 +1,7 @@
 (** Test heaps *)
 
 open OUnit
-
-let print_int_list l =
-  let b = Buffer.create 20 in
-  Format.bprintf b "@[<h>[%a]@]"
-    (Sequence.pp_seq ~sep:", " Format.pp_print_int)
-    (Sequence.of_list l);
-  Buffer.contents b
+open Helpers
 
 let test_empty () =
   let h = Heap.empty ~cmp:(fun x y -> x - y) in
diff --git a/tests/tests.ml b/tests/tests.ml
index 09e6a200..7c157b36 100644
--- a/tests/tests.ml
+++ b/tests/tests.ml
@@ -6,6 +6,7 @@ let suite =
   "all_tests" >:::
     [ Test_pHashtbl.suite;
       Test_heap.suite;
+      Test_graph.suite;
     ]
 
 let _ =