diff --git a/src/lib/uTop.ml b/src/lib/uTop.ml
index fcc2e58..d8b9eab 100644
--- a/src/lib/uTop.ml
+++ b/src/lib/uTop.ml
@@ -67,6 +67,7 @@ type syntax =
 let syntax, get_syntax, set_syntax = make_variable Normal
 let phrase_terminator, get_phrase_terminator, set_phrase_terminator = make_variable ";;"
 let auto_run_lwt, get_auto_run_lwt, set_auto_run_lwt = make_variable true
+let auto_run_async, get_auto_run_async, set_auto_run_async = make_variable true
 
 (* +-----------------------------------------------------------------+
    | Keywords                                                        |
diff --git a/src/lib/uTop.mli b/src/lib/uTop.mli
index f1f6533..6e6cac8 100644
--- a/src/lib/uTop.mli
+++ b/src/lib/uTop.mli
@@ -88,6 +88,28 @@ val get_auto_run_lwt : unit -> bool
 val set_auto_run_lwt : bool -> unit
   (** Modifies {!auto_run_lwt}. *)
 
+val auto_run_async : bool signal
+  (** If [true] (the default) toplevel Async expressions are
+      automatically run with in a separate thread with 
+      [Thread_safe.block_on_async_exn]. i.e. if you type:
+
+      {[
+        after (Time.Span.of_s 1.0)
+      ]}
+
+      this will be replaced by:
+
+      {[
+        Thread_safe.block_on_async_exn (fun () -> after (Time.Span.of_s 1.0))
+      ]}
+  *)
+
+val get_auto_run_async : unit -> bool
+  (** Returns the value of {!auto_run_async}. *)
+
+val set_auto_run_async : bool -> unit
+  (** Modifies {!auto_run_async}. *)
+
 (** {6 History} *)
 
 val history : LTerm_history.t
diff --git a/src/lib/uTop_main.ml b/src/lib/uTop_main.ml
index df7f4bf..f1f8647 100644
--- a/src/lib/uTop_main.ml
+++ b/src/lib/uTop_main.ml
@@ -227,6 +227,8 @@ let with_loc loc str = str
 #endif
 
 let longident_lwt_main_run = Longident.Ldot (Longident.Lident "Lwt_main", "run")
+let longident_async_main_run = Longident.parse "Async.Std.Thread_safe.block_on_async_exn"
+let longident_unit = Longident.Lident "()"
 
 let is_eval = function
   | { Parsetree.pstr_desc = Parsetree.Pstr_eval _ } -> true
@@ -241,6 +243,16 @@ let rec is_lwt_t typ =
     | _ ->
         false
 
+let rec is_async_t typ =
+  match typ.Types.desc with
+    | Types.Tlink typ ->
+        is_async_t typ
+    | Types.Tconstr (path, _, _) ->
+        let n = Path.name path in
+        (n = "Async_core.Deferred.t") || (n = "Async.Std.Deferred.t")
+    | _ ->
+        false
+
 #if ocaml_version >= (4, 0, 0)
 let str_items_of_typed_structure tstr = tstr.Typedtree.str_items
 let str_desc_of_typed_str_item tstr = tstr.Typedtree.str_desc
@@ -292,6 +304,56 @@ let insert_lwt_main_run phrase =
     | Parsetree.Ptop_dir _ ->
         phrase
 
+let insert_async_main_run phrase =
+  let wrap_unit loc e =
+    let open Parsetree in
+    let i = with_loc loc longident_unit in
+    let p = { ppat_desc = Ppat_construct (i, None, false); ppat_loc=loc } in
+    { pexp_desc=Pexp_function ("", None, [p,e]); pexp_loc=loc } in
+  match phrase with
+    | Parsetree.Ptop_def pstr ->
+        let env = !Toploop.toplevel_env in
+        let async_main_run_is_the_real_one =
+          try
+            let path, _ = Env.lookup_value longident_async_main_run env in
+            let rec is_persistent = function
+             | Path.Pident id -> Ident.persistent id
+             | Path.Pdot(t, _, _) -> is_persistent t
+             | Path.Papply(_, p) -> is_persistent p
+            in
+            is_persistent path
+          with Not_found ->
+            false
+        in
+        if async_main_run_is_the_real_one && List.exists is_eval pstr then
+          let tstr, _, _ = Typemod.type_structure env pstr Location.none in
+          let tstr = str_items_of_typed_structure tstr in
+          Parsetree.Ptop_def
+            (List.map2
+               (fun pstr_item tstr_item ->
+                  match pstr_item, str_desc_of_typed_str_item tstr_item with
+                    | { Parsetree.pstr_desc = Parsetree.Pstr_eval e; Parsetree.pstr_loc = loc },
+                      Typedtree.Tstr_eval { Typedtree.exp_type = typ } when is_async_t typ ->
+                        {
+                          Parsetree.pstr_desc =
+                            Parsetree.Pstr_eval {
+                              Parsetree.pexp_desc =
+                                Parsetree.Pexp_apply
+                                  ({ Parsetree.pexp_desc = Parsetree.Pexp_ident (with_loc loc longident_async_main_run);
+                                     Parsetree.pexp_loc = loc },
+                                   [("", (wrap_unit loc e))]);
+                              Parsetree.pexp_loc = loc;
+                            };
+                          Parsetree.pstr_loc = loc;
+                        }
+                    | _ ->
+                        pstr_item)
+               pstr tstr)
+        else
+          phrase
+    | Parsetree.Ptop_dir _ ->
+        phrase
+
 (* +-----------------------------------------------------------------+
    | Main loop                                                       |
    +-----------------------------------------------------------------+ *)
@@ -345,6 +407,8 @@ let rec loop term =
     | Some phrase ->
         (* Add Lwt_main.run to toplevel evals. *)
         let phrase = if UTop.get_auto_run_lwt () then insert_lwt_main_run phrase else phrase in
+        (* Add Async execution to toplevel evals. *)
+        let phrase = if UTop.get_auto_run_async () then insert_async_main_run phrase else phrase in
         (* Set the margin of standard formatters. *)
         let cols = (LTerm.size term).cols in
         update_margin Format.std_formatter cols;
@@ -564,6 +628,7 @@ module Emacs(M : sig end) = struct
           if add_to_history then LTerm_history.add UTop.history input;
           (* Add Lwt_main.run to toplevel evals. *)
           let phrase = if UTop.get_auto_run_lwt () then insert_lwt_main_run phrase else phrase in
+          let phrase = if UTop.get_auto_run_async () then insert_async_main_run phrase else phrase in
           try
             ignore (Toploop.execute_phrase true Format.std_formatter phrase)
           with exn ->