diff --git a/src/proto_alpha/lib_protocol/src/alpha_context.mli b/src/proto_alpha/lib_protocol/src/alpha_context.mli
index 173f1f252..544e675e9 100644
--- a/src/proto_alpha/lib_protocol/src/alpha_context.mli
+++ b/src/proto_alpha/lib_protocol/src/alpha_context.mli
@@ -243,6 +243,7 @@ module Script : sig
     | I_LOOP_LEFT
     | I_ADDRESS
     | I_CONTRACT
+    | I_ISNAT
     | T_bool
     | T_contract
     | T_int
diff --git a/src/proto_alpha/lib_protocol/src/michelson_v1_primitives.ml b/src/proto_alpha/lib_protocol/src/michelson_v1_primitives.ml
index 1cea252fa..efc5c04ed 100644
--- a/src/proto_alpha/lib_protocol/src/michelson_v1_primitives.ml
+++ b/src/proto_alpha/lib_protocol/src/michelson_v1_primitives.ml
@@ -96,6 +96,7 @@ type prim =
   | I_LOOP_LEFT
   | I_ADDRESS
   | I_CONTRACT
+  | I_ISNAT
   | T_bool
   | T_contract
   | T_int
@@ -220,6 +221,7 @@ let string_of_prim = function
   | I_LOOP_LEFT -> "LOOP_LEFT"
   | I_ADDRESS -> "ADDRESS"
   | I_CONTRACT -> "CONTRACT"
+  | I_ISNAT -> "ISNAT"
   | T_bool -> "bool"
   | T_contract -> "contract"
   | T_int -> "int"
@@ -325,6 +327,7 @@ let prim_of_string = function
   | "LOOP_LEFT" -> ok I_LOOP_LEFT
   | "ADDRESS" -> ok I_ADDRESS
   | "CONTRACT" -> ok I_CONTRACT
+  | "ISNAT" -> ok I_ISNAT
   | "bool" -> ok T_bool
   | "contract" -> ok T_contract
   | "int" -> ok T_int
@@ -474,6 +477,7 @@ let prim_encoding =
     ("LOOP_LEFT", I_LOOP_LEFT) ;
     ("ADDRESS", I_ADDRESS) ;
     ("CONTRACT", I_CONTRACT) ;
+    ("ISNAT", I_ISNAT) ;
     ("bool", T_bool) ;
     ("contract", T_contract) ;
     ("int", T_int) ;
diff --git a/src/proto_alpha/lib_protocol/src/michelson_v1_primitives.mli b/src/proto_alpha/lib_protocol/src/michelson_v1_primitives.mli
index 051e2a50d..d2a5efa3b 100644
--- a/src/proto_alpha/lib_protocol/src/michelson_v1_primitives.mli
+++ b/src/proto_alpha/lib_protocol/src/michelson_v1_primitives.mli
@@ -94,6 +94,7 @@ type prim =
   | I_LOOP_LEFT
   | I_ADDRESS
   | I_CONTRACT
+  | I_ISNAT
   | T_bool
   | T_contract
   | T_int
diff --git a/src/proto_alpha/lib_protocol/src/script_int_repr.ml b/src/proto_alpha/lib_protocol/src/script_int_repr.ml
index 10213ee93..27e638bc3 100644
--- a/src/proto_alpha/lib_protocol/src/script_int_repr.ml
+++ b/src/proto_alpha/lib_protocol/src/script_int_repr.ml
@@ -43,6 +43,8 @@ let mul_n = mul
 let ediv_n = ediv
 
 let abs x = Z.abs x
+let is_nat x =
+  if Compare.Z.(x < Z.zero) then None else Some x
 let neg x = Z.neg x
 let int x = x
 
diff --git a/src/proto_alpha/lib_protocol/src/script_int_repr.mli b/src/proto_alpha/lib_protocol/src/script_int_repr.mli
index 2765b62ed..6f01e4d95 100644
--- a/src/proto_alpha/lib_protocol/src/script_int_repr.mli
+++ b/src/proto_alpha/lib_protocol/src/script_int_repr.mli
@@ -88,6 +88,9 @@ val ediv:  _ num -> _ num -> (z num * n num) option
 (** Compute the absolute value of a relative, turning it into a natural. *)
 val abs : z num -> n num
 
+(** Partial identity over [N]. *)
+val is_nat : z num -> n num option
+
 (** Negates a number. *)
 val neg : _ num -> z num
 
diff --git a/src/proto_alpha/lib_protocol/src/script_interpreter.ml b/src/proto_alpha/lib_protocol/src/script_interpreter.ml
index 0632f02cd..12b7da87d 100644
--- a/src/proto_alpha/lib_protocol/src/script_interpreter.ml
+++ b/src/proto_alpha/lib_protocol/src/script_interpreter.ml
@@ -451,6 +451,8 @@ let rec interp
         | Not, Item (x, rest) ->
             consume_gas_unop descr (not, x) Interp_costs.bool_unop rest ctxt
         (* integer operations *)
+        | Is_nat, Item (x, rest) ->
+            consume_gas_unop descr (Script_int.is_nat, x) Interp_costs.abs rest ctxt
         | Abs_int, Item (x, rest) ->
             consume_gas_unop descr (Script_int.abs, x) Interp_costs.abs rest ctxt
         | Int_nat, Item (x, rest) ->
diff --git a/src/proto_alpha/lib_protocol/src/script_ir_translator.ml b/src/proto_alpha/lib_protocol/src/script_ir_translator.ml
index beffd7f80..d203865ee 100644
--- a/src/proto_alpha/lib_protocol/src/script_ir_translator.ml
+++ b/src/proto_alpha/lib_protocol/src/script_ir_translator.ml
@@ -151,6 +151,7 @@ let number_of_generated_growing_types : type b a. (b, a) instr -> int = function
   | And -> 0
   | Xor -> 0
   | Not -> 0
+  | Is_nat -> 0
   | Neg_nat -> 0
   | Neg_int -> 0
   | Abs_int -> 0
@@ -306,7 +307,8 @@ let namespace = function
   | I_ITER
   | I_LOOP_LEFT
   | I_ADDRESS
-  | I_CONTRACT -> Instr_namespace
+  | I_CONTRACT
+  | I_ISNAT -> Instr_namespace
   | T_bool
   | T_contract
   | T_int
@@ -1845,6 +1847,14 @@ and parse_instr
       Item_t (Int_t, rest, _) ->
         typed ctxt loc Abs_int
           (Item_t (Nat_t, rest, instr_annot))
+    | Prim (loc, I_ISNAT, [], Some instr_annot),
+      Item_t (Int_t, rest, None) ->
+        typed ctxt loc Is_nat
+          (Item_t (Option_t Nat_t, rest, Some instr_annot))
+    | Prim (loc, I_ISNAT, [], None),
+      Item_t (Int_t, rest, annot) ->
+        typed ctxt loc Is_nat
+          (Item_t (Option_t Nat_t, rest, annot))
     | Prim (loc, I_INT, [], instr_annot),
       Item_t (Nat_t, rest, _) ->
         typed ctxt loc Int_nat
diff --git a/src/proto_alpha/lib_protocol/src/script_typed_ir.ml b/src/proto_alpha/lib_protocol/src/script_typed_ir.ml
index 3e935c9f7..a89c378ae 100644
--- a/src/proto_alpha/lib_protocol/src/script_typed_ir.ml
+++ b/src/proto_alpha/lib_protocol/src/script_typed_ir.ml
@@ -232,6 +232,8 @@ and ('bef, 'aft) instr =
   | Not :
       (bool * 'rest, bool * 'rest) instr
   (* integer operations *)
+  | Is_nat :
+      (z num * 'rest, n num option * 'rest) instr
   | Neg_nat :
       (n num * 'rest, z num * 'rest) instr
   | Neg_int :