diff --git a/src/fib/fiber.ml b/src/fib/fiber.ml
index 4f776c80..7fd11bf9 100644
--- a/src/fib/fiber.ml
+++ b/src/fib/fiber.ml
@@ -265,7 +265,7 @@ let spawn_top ~on f : _ t =
   let ls = Task_local_storage.Direct.create () in
   spawn_ ~ls ~runner:on ~parent:None f
 
-let spawn ?(protect = true) f : _ t =
+let spawn ?on ?(protect = true) f : _ t =
   (* spawn [f()] with a copy of our local storage *)
   let (Any p) =
     match get_cur () with
@@ -273,7 +273,12 @@ let spawn ?(protect = true) f : _ t =
     | Some p -> p
   in
   let ls = Task_local_storage.Direct.copy p.ls in
-  let child = spawn_ ~ls ~parent:(Some p) ~runner:p.runner f in
+  let runner =
+    match on with
+    | Some r -> r
+    | None -> p.runner
+  in
+  let child = spawn_ ~ls ~parent:(Some p) ~runner f in
   add_child_ ~protect p child;
   child
 
diff --git a/src/fib/fiber.mli b/src/fib/fiber.mli
index 25787cc1..d02c4e56 100644
--- a/src/fib/fiber.mli
+++ b/src/fib/fiber.mli
@@ -120,12 +120,14 @@ val spawn_top : on:Runner.t -> (unit -> 'a) -> 'a t
     This fiber is not the child of any other fiber: its lifetime
     is only determined by the lifetime of [f()]. *)
 
-val spawn : ?protect:bool -> (unit -> 'a) -> 'a t
+val spawn : ?on:Runner.t -> ?protect:bool -> (unit -> 'a) -> 'a t
 (** [spawn ~protect f] spawns a sub-fiber [f_child]
     from a running fiber [parent].
     The sub-fiber [f_child] is attached to the current fiber and fails
     if the current fiber [parent] fails.
 
+    @param on if provided, start the fiber on the given runner. If not
+    provided, use the parent's runner.
     @param protect if true, when [f_child] fails, it does not
       affect [parent]. If false, [f_child] failing also
       causes [parent] to fail (and therefore all other children