diff --git a/compiler/heptagon/analysis/async_check.ml b/compiler/heptagon/analysis/async_check.ml
new file mode 100644
index 0000000..a2a33a8
--- /dev/null
+++ b/compiler/heptagon/analysis/async_check.ml
@@ -0,0 +1,57 @@
+open Names
+open Location
+open Heptagon
+open Hept_mapfold
+
+type error =
+  | Etoo_much_async
+
+let message loc kind =
+  begin match kind with
+    | Etoo_much_async ->
+        Format.eprintf "%aInvalid async nesting.@."
+          print_location loc
+  end;
+  raise Errors.Error
+
+(* Compute the set of nodes that use at least one async call. *)
+let exp_callers funs (callers, current) e =
+  let e, (callers, current) = Hept_mapfold.exp funs (callers, current) e in
+  match e.e_desc with
+    | Eapp({ a_op = Easync _ }, _, _) ->
+      e, (QualSet.add (Option.get current) callers, current)
+    (* TODO(Arduino): Eiterator *)
+    | _ -> e, (callers, current)
+
+let node_dec_callers funs (callers, _) n =
+  Hept_mapfold.node_dec funs (callers, Some n.n_name) n
+
+let funs_callers =
+  { Hept_mapfold.defaults with
+      node_dec = node_dec_callers;
+      exp = exp_callers }
+
+(* Ensure that no node using an async call is called more than once. *)
+let exp_async funs (callers, calls) e =
+  let e, (callers, calls) = Hept_mapfold.exp funs (callers, calls) e in
+  match e.e_desc with
+    | Eapp({ a_op = Easync (name, _) }, _, _) ->
+      let caller = QualSet.mem name callers in
+      let exists = QualSet.mem name calls in
+      if caller && exists then
+        message e.e_loc Etoo_much_async
+      else
+        e, (callers, QualSet.add name calls)
+    (* TODO(Arduino): Eiterator *)
+    | _ -> e, (callers, calls)
+
+let funs_async =
+  { Hept_mapfold.defaults with
+      exp = exp_async }
+
+let program p =
+  let _, (callers, _) =
+    Hept_mapfold.program_it funs_callers (QualSet.empty, None) p
+  in
+  let _ = Hept_mapfold.program_it funs_async (callers, QualSet.empty) p in
+  p
diff --git a/compiler/heptagon/main/hept_compiler.ml b/compiler/heptagon/main/hept_compiler.ml
index 4f299d1..ca6c284 100644
--- a/compiler/heptagon/main/hept_compiler.ml
+++ b/compiler/heptagon/main/hept_compiler.ml
@@ -37,6 +37,7 @@ let compile_program p log_c =
   (* Typing *)
   let p = silent_pass "Statefulness check" true Stateful.program p in
   let p = silent_pass "Unsafe check" true Unsafe.program p in
+  let p = silent_pass "Async check" true Async_check.program p in
   let p = pass "Typing" true Typing.program p pp in
   let p = pass "Linear Typing" !do_linear_typing Linear_typing.program p pp in