diff --git a/lib/check.ml b/lib/check.ml
index 49711623..6957088c 100644
--- a/lib/check.ml
+++ b/lib/check.ml
@@ -3744,7 +3744,8 @@ and in_range loc k n =
     else if bits = 64 then
       (* A u64 literal is its 64-bit pattern, so anything at or above 2^63
          arrives here as a negative [int64] and is still in range —
-         0xcbf29ce484222325 is a real u64 and not an error. The cost is that a
+         0xcbf29ce484222325 is a real u64 and not an error, and so is its
+         decimal, which the reader reads as the same pattern. The cost is that a
          negative *decimal* literal is accepted as a u64 too, because the
          reader records only the value and not how it was written. Narrower
          unsigned types keep the strict check, which is where a typo like 300
@@ -8796,7 +8797,18 @@ and named_call ?(qualified = false) ctx ~want loc name args =
     prim (Tast.Cast target) target [ a ]
   | _ when is_cast name && List.length args = 1 ->
     let target = resolve_name ctx.env ~seen:[] loc name in
-    let a = check ctx (List.hd args) in
+    (* An integer literal too wide for the i32 it would default to is checked
+       at the target instead, so (u64 2935910691) and (i64 5000000000) are the
+       constants they say. One that fits i32 keeps the default and the cast,
+       which is what (u32 -1) has always meant. *)
+    let want =
+      match (List.hd args).Ast.e, target with
+      | Ast.Int n, (Types.Int _ | Types.Float _)
+        when Int64.compare n (-2147483648L) < 0
+             || Int64.compare n 2147483647L > 0 -> Some target
+      | _ -> None
+    in
+    let a = check ctx ?want (List.hd args) in
     (match a.Tast.ty with
      | Types.Enum _ -> ()
      (* A dyn opens here — [cast_dyn], TODO.org, "A numeric cast opens a dyn
diff --git a/lib/reader.ml b/lib/reader.ml
index 0e098a79..eee007a5 100644
--- a/lib/reader.ml
+++ b/lib/reader.ml
@@ -153,10 +153,22 @@ let read_number st =
     | None -> Loc.failk "reader/malformed-number" (Loc.upto loc (here st))
         "malformed float literal %s" text
   else
+    (* A decimal above the largest i64 and below 2^64 is read as its 64-bit
+       pattern, as a hex literal is, so a u64 constant can be written in
+       decimal. It therefore arrives negative, and [Check.in_range] makes the
+       same allowance for it that it makes for hex. *)
+    let unsigned () =
+      if String.for_all (fun c -> c >= '0' && c <= '9') text then
+        Int64.of_string_opt ("0u" ^ text)
+      else None
+    in
     match Int64.of_string_opt text with
     | Some i -> spanned st loc (Form.Int i)
-    | None -> Loc.failk "reader/malformed-number" (Loc.upto loc (here st))
-        "malformed integer literal %s" text
+    | None ->
+      match unsigned () with
+      | Some i -> spanned st loc (Form.Int i)
+      | None -> Loc.failk "reader/malformed-number" (Loc.upto loc (here st))
+          "malformed integer literal %s" text
 
 let read_symbol_or_keyword st =
   let loc = here st in
diff --git a/test/programs/u64-decimal.flan b/test/programs/u64-decimal.flan
new file mode 100644
index 00000000..00b0c92a
--- /dev/null
+++ b/test/programs/u64-decimal.flan
@@ -0,0 +1,16 @@
+;;;; A u64 constant above 2^63 written in decimal, and an integer literal too
+;;;; wide for i32 as a cast's argument.
+
+(defconst top u64 18446744073709551615)
+(defonce fnv u64 14695981039346656037)
+
+(defn main [] i32
+  (println top)
+  (println fnv)
+  (println (u64 2935910691))
+  (println (u64 18446744073709551615))
+  (println (i64 -5000000000))
+  (println (f64 4000000000))
+  ;; A literal that fits i32 keeps its default and is cast, as before.
+  (println (u32 -1))
+  0)
diff --git a/test/test_acceptance.ml b/test/test_acceptance.ml
index d1163867..745846d8 100644
--- a/test/test_acceptance.ml
+++ b/test/test_acceptance.ml
@@ -508,6 +508,14 @@ let () =
       "2 3 4 7 9 3\n";
     outputs ~x86:true "vec-new takes a type expression, x86"
       "programs/vec-new-type.flan" "2 3 4 7 9 3\n";
+    (* A u64 above 2^63 in decimal, and a cast of a literal too wide for i32. *)
+    let u64_out =
+      "18446744073709551615\n14695981039346656037\n2935910691\n\
+       18446744073709551615\n-5000000000\n4e+09\n4294967295\n"
+    in
+    outputs "a u64 constant in decimal" "programs/u64-decimal.flan" u64_out;
+    outputs ~x86:true "a u64 constant in decimal, x86"
+      "programs/u64-decimal.flan" u64_out;
     (* into. The count of pulls is the assertion a unit test cannot make: one
        pass, one call per element per stage it reaches, and no intermediate
        collection anywhere. The two show lines either side of it are the same
diff --git a/test/test_flan.ml b/test/test_flan.ml
index ff0cf47d..fa77e13c 100644
--- a/test/test_flan.ml
+++ b/test/test_flan.ml
@@ -92,6 +92,13 @@ let () =
   reads "bare plus"      "+" "+";
   reads "float"          "0.05" "0.05";
   reads "hex"            "0xE6B800FF" "3870818559";
+  (* A decimal between 2^63 and 2^64 is its bit pattern, as hex is; one past
+     2^64, or a negative one past the smallest i64, is still malformed. *)
+  reads "u64 decimal"    "18446744073709551615" "-1";
+  rejects "decimal past 2^64" "18446744073709551616"
+    ~needle:"malformed integer literal";
+  rejects "negative decimal past i64" "-9223372036854775809"
+    ~needle:"malformed integer literal";
   reads "string"         "\"SAND\"" "\"SAND\"";
   reads "symbol"         "empty-at?" "empty-at?";
   reads "qualified"      "rl/draw-fps" "rl/draw-fps";