diff --git a/compiler/wasm/wasm_translate.ml b/compiler/wasm/wasm_translate.ml
index d6aef48a2..2fce7735a 100644
--- a/compiler/wasm/wasm_translate.ml
+++ b/compiler/wasm/wasm_translate.ml
@@ -286,13 +286,15 @@ module Constants = struct
       ; i32_const' 5 (* string *)
       ; eq i32
       ; if_
-          [ i32_const' 0
-          ; new_ [] "EjStringBuilder"
-          ; local_set foreign_ptr
-          (* load length / characters to be processed *)
-          ; local_get local_ptr
+          [ (* load length / characters to be processed *)
+            local_get local_ptr
           ; load ctx.ctx.memory ~offset:8 i32
           ; local_set n
+          (* initialize buffer in runtime *)
+          ; i32_const' 0
+          ; local_get n
+          ; new_ [i32] "EjStringBuilderUTF8"
+          ; local_set foreign_ptr
           (* initialize moving pointer *)
           ; local_get local_ptr
           ; i32_const' 12
@@ -304,7 +306,7 @@ module Constants = struct
             ; if_
               [ local_get foreign_ptr (* for release *)
               ; local_get foreign_ptr (* for finalize *)
-              ; call (foreign ~params:[i32] ~result:[i32] "EjStringBuilder#finalize")
+              ; call (foreign ~params:[i32] ~result:[i32] "EjStringBuilderUTF8#finalize")
               ; local_set foreign_ptr (* for return *)
               ; call (foreign ~params:[i32] ~result:[] "__release")
               ; local_get foreign_ptr
@@ -316,9 +318,9 @@ module Constants = struct
             ; local_set n
             ; local_get foreign_ptr
             ; local_get local_ptr
+            (* TODO: speed up string constant allocation by copying words. *)
             ; load ctx.ctx.memory ~pack:U8 i32
-            ; call (foreign ~params:[i32; i32] ~result:[i32] "EjStringBuilder#append")
-            ; drop
+            ; call (foreign ~params:[i32; i32] ~result:[] "EjStringBuilderUTF8#putByte")
             ; local_get local_ptr
             ; i32_const' 1
             ; add i32
diff --git a/runtimes/assemblyscript/assembly/index.ts b/runtimes/assemblyscript/assembly/index.ts
index 08557fa0f..ca187ca16 100644
--- a/runtimes/assemblyscript/assembly/index.ts
+++ b/runtimes/assemblyscript/assembly/index.ts
@@ -38,24 +38,19 @@ export class EjString extends EjValue {
 }
 export const IdEjString = idof<EjString>()
 
-export class EjStringBuilder {
-  private buf: Array<i32>
+export class EjStringBuilderUTF8 {
+  private buf: Uint8Array
   private pos: i32
-  constructor() {
+  constructor(n: i32) {
     this.pos = 0;
-    this.buf = new Array<i32>(32);
+    this.buf = new Uint8Array(n);
   }
-  append(val: i32): EjStringBuilder {
-    if (this.pos >= this.buf.length) {
-      this.buf = this.buf.concat(new Array(this.buf.length * 2));
-    }
+  putByte(val: u8): void {
     this.buf[this.pos] = val;
     this.pos++;
-    return this;
   }
   finalize(): EjString {
-    let buf = this.buf.slice(0, this.pos);
-    let str = String.fromCharCodes(buf);
+    let str = String.UTF8.decode(this.buf.buffer);
     return new EjString(str);
   }
 }