diff --git a/src/bin_client/test/contracts/big_map_union.tz b/src/bin_client/test/contracts/big_map_union.tz
new file mode 100644
index 000000000..953b66ce9
--- /dev/null
+++ b/src/bin_client/test/contracts/big_map_union.tz
@@ -0,0 +1,9 @@
+parameter (list (pair string int)) ;
+storage (pair (big_map string int) unit) ;
+return unit ;
+code { UNPAAIAIR ;
+       ITER { UNPAIR ; DUUUP ; DUUP; GET ;
+              IF_NONE { PUSH int 0 } {} ;
+              SWAP ; DIP { ADD ; SOME } ;
+              UPDATE } ;
+       PAIR ; UNIT ; PAIR }
diff --git a/src/proto_alpha/lib_protocol/test/helpers/helpers_operation.ml b/src/proto_alpha/lib_protocol/test/helpers/helpers_operation.ml
index 262677499..a5d09c5a5 100644
--- a/src/proto_alpha/lib_protocol/test/helpers/helpers_operation.ml
+++ b/src/proto_alpha/lib_protocol/test/helpers/helpers_operation.ml
@@ -31,7 +31,7 @@ let manager_full src ?(fee = Tez.zero) ops context =
   manager src ~fee ops context >>=? fun ops -> return @@ sourced ops
 
 
-let transaction ?(parameters = None) amount destination =
+let transaction ?parameters amount destination =
   Transaction {
     amount ;
     parameters ;
@@ -70,8 +70,8 @@ let origination_full ?(spendable = true) ?(delegatable = true) ?(fee = Tez.zero)
   manager_full src ~fee [origination ~spendable ~delegatable src credit] context
 
 
-let transaction_full ?(fee = Tez.zero) src dst amount context =
-  manager src ~fee [transaction amount dst] context
+let transaction_full ?(fee = Tez.zero) ?parameters src dst amount context =
+  manager src ~fee [transaction ?parameters amount dst] context
   >>=? fun manager_op ->
   return @@ sourced manager_op
 
diff --git a/src/proto_alpha/lib_protocol/test/helpers/helpers_operation.mli b/src/proto_alpha/lib_protocol/test/helpers/helpers_operation.mli
index dceea14c4..045880bcc 100644
--- a/src/proto_alpha/lib_protocol/test/helpers/helpers_operation.mli
+++ b/src/proto_alpha/lib_protocol/test/helpers/helpers_operation.mli
@@ -23,7 +23,7 @@ val manager_full :
   Proto_alpha.Environment.Context.t -> proto_operation proto_tzresult Lwt.t
 
 val transaction :
-  ?parameters:Script.expr option -> Tez.t -> Contract.contract ->
+  ?parameters:Script.expr -> Tez.t -> Contract.contract ->
   manager_operation
 
 val origination :
@@ -46,7 +46,7 @@ val origination_full :
   proto_operation proto_tzresult Lwt.t
 
 val transaction_full :
-  ?fee:Tez.tez -> Helpers_account.t -> Contract.contract -> Tez.t ->
+  ?fee:Tez.tez -> ?parameters:Proto_alpha.Tezos_context.Script.expr -> Helpers_account.t -> Contract.contract -> Tez.t ->
   Proto_alpha.Environment.Context.t -> proto_operation proto_tzresult Lwt.t
 
 val delegate :
diff --git a/src/proto_alpha/lib_protocol/test/main.ml b/src/proto_alpha/lib_protocol/test/main.ml
index 8aa94d7d7..3d8c2c9e7 100644
--- a/src/proto_alpha/lib_protocol/test/main.ml
+++ b/src/proto_alpha/lib_protocol/test/main.ml
@@ -14,4 +14,5 @@ let () =
       Test_transaction.tests @
       Test_endorsement.tests @
       Test_origination.tests @
+      Test_big_maps.tests @
       Test_michelson.tests )
diff --git a/src/proto_alpha/lib_protocol/test/test_big_maps.ml b/src/proto_alpha/lib_protocol/test/test_big_maps.ml
new file mode 100644
index 000000000..bb11f6e10
--- /dev/null
+++ b/src/proto_alpha/lib_protocol/test/test_big_maps.ml
@@ -0,0 +1,132 @@
+(**************************************************************************)
+(*                                                                        *)
+(*    Copyright (c) 2014 - 2016.                                          *)
+(*    Dynamic Ledger Solutions, Inc. <contact@tezos.com>                  *)
+(*                                                                        *)
+(*    All rights reserved. No warranty, explicit or implicit, provided.   *)
+(*                                                                        *)
+(**************************************************************************)
+
+
+let name = "Isolate Big Maps"
+module Logger = Logging.Make(struct let name = name end)
+let section = Lwt_log.Section.make name
+let () =
+  Lwt_log.Section.set_level section Lwt_log.Debug(*.Warning*)
+
+open Logger
+
+module Helpers = Isolate_helpers
+module Assert = Helpers.Assert
+
+let (>>??) = Helpers.Assert.(>>??)
+let (>>=??) = Helpers.Assert.(>>=??)
+
+let parse_expr s : Proto_alpha.Tezos_context.Script.expr tzresult =
+  Micheline_parser.no_parsing_error (Michelson_v1_parser.parse_expression s) >>? fun parsed ->
+  ok parsed.expanded
+
+let parse_script code_str storage_str : Proto_alpha.Tezos_context.Script.t tzresult =
+  parse_expr code_str >>? fun code ->
+  parse_expr storage_str >>? fun storage ->
+  ok { Proto_alpha.Tezos_context.Script.code ; storage }
+
+let code = {|
+{ parameter (list (pair string int)) ;
+  storage (pair (big_map string int) unit) ;
+  return unit ;
+  code { UNPAAIAIR ;
+         ITER { UNPAIR ; DUUUP ; DUUP; GET ;
+                IF_NONE { PUSH int 0 } {} ;
+                SWAP ; DIP { ADD ; SOME } ;
+                UPDATE } ;
+         PAIR ; UNIT ; PAIR } }
+|}
+
+let storage = {| Pair { Elt "A" 1 ; Elt "B" 2 } Unit |}
+
+let expect_big_map tc contract print_key key_type print_data data_type contents =
+  let open Proto_alpha.Error_monad in
+  iter_p
+    (fun (n, exp) ->
+       let key = Proto_alpha.Script_ir_translator.hash_data key_type n in
+       Proto_alpha.Tezos_context.Contract.Big_map_storage.get_opt tc contract key >>=? fun data ->
+       match data, exp with
+       | None, None ->
+           debug " - big_map[%a] is not defined (ok)" print_key n ;
+           return ()
+       | None, Some _ ->
+           debug " - big_map[%a] is not defined (error)" print_key n ;
+           Helpers_assert.fail_msg "Wrong big map contents"
+       | Some data, None ->
+           Proto_alpha.Script_ir_translator.parse_data tc data_type (Micheline.root data) >>=? fun data ->
+           debug " - big_map[%a] = %a (error)" print_key n print_data data ;
+           Helpers_assert.fail_msg "Wrong big map contents"
+       | Some data, Some exp ->
+           Proto_alpha.Script_ir_translator.parse_data tc data_type (Micheline.root data) >>=? fun data ->
+           debug " - big_map[%a] = %a (expected %a)" print_key n print_data data print_data exp ;
+           Helpers_assert.equal data exp ;
+           return ())
+    contents
+
+let main () =
+  Helpers.Init.main () >>=?? fun pred ->
+  let tc = pred.Helpers.Block.tezos_context in
+  let src = List.hd Helpers.Account.bootstrap_accounts in
+  Lwt.return (parse_script code storage) >>=? fun script ->
+  Helpers.Apply.script_origination_pred
+    ~tc ~pred (script , src, 100_00) >>=?? fun ((contracts, errs), tc) ->
+  begin match errs with
+    | None | Some [] -> Proto_alpha.Error_monad.return ()
+    | Some (err :: _) -> Proto_alpha.Error_monad.fail err
+  end >>=?? fun () ->
+  begin match contracts with
+    | [ contract ] -> return contract
+    | _ -> failwith "more than one contract"
+  end >>=? fun contract ->
+  debug "contract created" ;
+  let expect_big_map tc exp =
+    expect_big_map tc contract
+      (fun ppf k -> Format.fprintf ppf "%s" k)
+      Proto_alpha.Script_typed_ir.String_t
+      (fun ppf n -> Format.fprintf ppf "%s" (Proto_alpha.Tezos_context.Script_int.to_string n))
+      Proto_alpha.Script_typed_ir.Int_t
+      (List.map (fun (n, v) -> (n, Option.map ~f:Proto_alpha.Tezos_context.Script_int.of_int v)) exp) in
+  expect_big_map tc
+    [ "A", Some 1 ; "B", Some 2 ; "C", None ; "D", None ] >>=?? fun () ->
+  debug "initial big map is ok" ;
+  let call tc input result =
+    Lwt.return (parse_expr input) >>=? fun parameters ->
+    Helpers.Operation.transaction_full
+      src contract (Helpers_cast.cents_of_int 100_00)
+      (Helpers_cast.ctxt_of_tc tc)
+      ~parameters >>=?? fun op ->
+    Helpers.Apply.operation ~tc
+      ~src pred.Helpers_block.hash
+      (Helpers_block.get_op_header_res pred)
+      op >>=?? fun ((_, errs), tc) ->
+    begin match errs with
+      | None | Some [] -> Proto_alpha.Error_monad.return ()
+      | Some (err :: _) -> Proto_alpha.Error_monad.fail err
+    end >>=?? fun () ->
+    expect_big_map tc result >>=?? fun () ->
+    debug "big map after call %s is ok" input ;
+    return tc in
+  call tc
+    {| {} |}
+    [ "A", Some 1 ; "B", Some 2 ; "C", None ; "D", None ] >>=? fun tc ->
+  call tc
+    {| { Pair "A" 2 } |}
+    [ "A", Some 3 ; "B", Some 2 ; "C", None ; "D", None ] >>=? fun tc ->
+  call tc
+    {| { Pair "A" 2 ; Pair "A" 2 ; Pair "D" 8 } |}
+    [ "A", Some 7 ; "B", Some 2 ; "C", None ; "D", Some 8 ] >>=? fun tc ->
+  call tc
+    {| { Pair "C" 3 } |}
+    [ "A", Some 7 ; "B", Some 2 ; "C", Some 3 ; "D", Some 8 ] >>=? fun _tc ->
+  Error_monad.return ()
+
+
+let tests = [
+  "main", (fun _ -> main ()) ;
+]