diff --git a/sequence.ml b/sequence.ml
index 784c546..e5ddf09 100644
--- a/sequence.ml
+++ b/sequence.ml
@@ -231,8 +231,9 @@ end
 
 (** Iterate on the sequence, storing elements in a data structure.
     The resulting sequence can be iterated on as many times as needed. *)
-let persistent (seq : 'a t) : 'a t =
-  let l = MList.of_seq seq in
+let persistent ?(blocksize=64) seq =
+  if blocksize < 2 then failwith "Sequence.persistent: blocksize too small";
+  let l = MList.of_seq ~size:blocksize seq in
   from_iter (fun k -> MList.iter k l)
 
 (** Sort the sequence. Eager, O(n) ram and O(n ln(n)) time. *)
diff --git a/sequence.mli b/sequence.mli
index e8ef972..86b6ba3 100644
--- a/sequence.mli
+++ b/sequence.mli
@@ -151,11 +151,15 @@ val fmap : ('a -> 'b option) -> 'a t -> 'b t
 val intersperse : 'a -> 'a t -> 'a t
   (** Insert the single element between every element of the sequence *)
 
-val persistent : 'a t -> 'a t
+val persistent : ?blocksize:int -> 'a t -> 'a t
   (** Iterate on the sequence, storing elements in a data structure.
       The resulting sequence can be iterated on as many times as needed.
       {b Note}: calling persistent on an already persistent sequence
-      will still make a new copy of the sequence! *)
+      will still make a new copy of the sequence!
+      
+      @param blocksize the size of chunks in the unrolled list
+        used to store elements. Use bigger values for bigger sequences.
+        Default: 64 *)
 
 val sort : ?cmp:('a -> 'a -> int) -> 'a t -> 'a t
   (** Sort the sequence. Eager, O(n) ram and O(n ln(n)) time.