diff --git a/include/picrin.h b/include/picrin.h
index eb42720c..e125260f 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -36,6 +36,7 @@ extern "C" {
 
 #include "xhash/xhash.h"
 #include "xfile/xfile.h"
+#include "xrope/xrope.h"
 
 #if __STDC_VERSION__ >= 201112L
 # define NORETURN _Noreturn
diff --git a/include/picrin/string.h b/include/picrin/string.h
index da342bd5..c2564ffe 100644
--- a/include/picrin/string.h
+++ b/include/picrin/string.h
@@ -11,8 +11,7 @@ extern "C" {
 
 struct pic_string {
   PIC_OBJECT_HEADER
-  char *str;
-  size_t len;
+  xrope *rope;
 };
 
 #define pic_str_p(v) (pic_type(v) == PIC_TT_STRING)
@@ -30,6 +29,8 @@ pic_str *pic_strcat(pic_state *, pic_str *, pic_str *);
 pic_str *pic_substr(pic_state *, pic_str *, size_t, size_t);
 int pic_strcmp(pic_str *, pic_str *);
 
+const char *pic_str_cstr(pic_str *);
+
 pic_value pic_format(pic_state *, const char *, ...);
 pic_value pic_vformat(pic_state *, const char *, va_list);
 pic_value pic_vfformat(pic_state *, xFILE *, const char *, va_list);
diff --git a/src/error.c b/src/error.c
index 81cc3403..0522bc1b 100644
--- a/src/error.c
+++ b/src/error.c
@@ -16,7 +16,7 @@ pic_errmsg(pic_state *pic)
 {
   assert(pic->err != NULL);
 
-  return pic->err->msg->str;
+  return pic_str_cstr(pic->err->msg);
 }
 
 NORETURN static void
diff --git a/src/gc.c b/src/gc.c
index 26d81341..016b448e 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -595,7 +595,7 @@ gc_finalize_object(pic_state *pic, struct pic_object *obj)
     break;
   }
   case PIC_TT_STRING: {
-    pic_free(pic, (void*)((struct pic_string *)obj)->str);
+    XROPE_DECREF(((struct pic_string *)obj)->rope);
     break;
   }
   case PIC_TT_PORT: {
diff --git a/src/macro.c b/src/macro.c
index 3b0642ea..13a760e8 100644
--- a/src/macro.c
+++ b/src/macro.c
@@ -619,11 +619,11 @@ pic_macro_include(pic_state *pic)
   body = pic_list(pic, 1, pic_symbol_value(pic->sBEGIN));
 
   for (i = 0; i < argc; ++i) {
-    char *filename;
+    const char *filename;
     if (! pic_str_p(argv[i])) {
       pic_error(pic, "expected string");
     }
-    filename = pic_str_ptr(argv[i])->str;
+    filename = pic_str_cstr(pic_str_ptr(argv[i]));
     file = fopen(filename, "r");
     if (file == NULL) {
       pic_error(pic, "could not open file");
diff --git a/src/parse.y b/src/parse.y
index 7374b51b..3f615887 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -94,8 +94,7 @@ yy_str_new_cstr(struct parser_control *p, const char *cstr)
   struct pic_string *str;
 
   str = (struct pic_string *)yy_obj_alloc(p, sizeof(struct pic_string), PIC_TT_STRING);
-  str->len = strlen(cstr);
-  str->str = pic_strdup(p->pic, cstr);
+  str->rope = xr_new_volatile(cstr, strlen(cstr));
 
   return pic_obj_value(str);
 }
diff --git a/src/port.c b/src/port.c
index ae40285a..84f60297 100644
--- a/src/port.c
+++ b/src/port.c
@@ -414,7 +414,7 @@ pic_port_read_line(pic_state *pic)
   }
 
   str = pic_get_output_string(pic, buf);
-  if (str->len == 0 && c == EOF) {
+  if (pic_strlen(str) == 0 && c == EOF) {
     return pic_eof_object();
   }
   else {
diff --git a/src/string.c b/src/string.c
index 2bcdce92..9dc31975 100644
--- a/src/string.c
+++ b/src/string.c
@@ -9,24 +9,25 @@
 #include "picrin/pair.h"
 #include "picrin/port.h"
 
-pic_str *
-pic_str_new(pic_state *pic, const char *cstr, size_t len)
+static pic_str *
+str_new_rope(pic_state *pic, xrope *rope)
 {
   pic_str *str;
-  char *copy;
-
-  if (cstr) {
-    copy = pic_strdup(pic, cstr);
-  } else {
-    copy = (char *)pic_alloc(pic, len);
-  }
 
   str = (pic_str *)pic_obj_alloc(pic, sizeof(pic_str), PIC_TT_STRING);
-  str->len = len;
-  str->str = copy;
+  str->rope = rope;             /* delegate ownership */
   return str;
 }
 
+pic_str *
+pic_str_new(pic_state *pic, const char *imbed, size_t len)
+{
+  if (imbed == NULL && len > 0) {
+    pic_errorf(pic, "zero length specified against NULL ptr");
+  }
+  return str_new_rope(pic, xr_new_volatile(imbed, len));
+}
+
 pic_str *
 pic_str_new_cstr(pic_state *pic, const char *cstr)
 {
@@ -38,73 +39,100 @@ pic_str_new_fill(pic_state *pic, size_t len, char fill)
 {
   size_t i;
   char *cstr;
+  pic_str *str;
 
   cstr = (char *)pic_alloc(pic, len + 1);
+  cstr[len] = '\0';
   for (i = 0; i < len; ++i) {
     cstr[i] = fill;
   }
-  cstr[len] = '\0';
 
-  return pic_str_new(pic, cstr, len);
+  str = pic_str_new(pic, cstr, len);
+
+  pic_free(pic, cstr);
+  return str;
 }
 
 size_t
 pic_strlen(pic_str *str)
 {
-  return str->len;
+  return xr_len(str->rope);
 }
 
 char
-pic_str_ref(pic_state *pic, pic_str *str, size_t n)
+pic_str_ref(pic_state *pic, pic_str *str, size_t i)
 {
-  UNUSED(pic);
+  char c;
 
-  return str->str[n];
+  c = xr_at(str->rope, i);
+  if (c == -1) {
+    pic_errorf(pic, "index out of range %d", i);
+  }
+  return c;
+}
+
+static xrope *
+xr_put(xrope *rope, size_t i, char c)
+{
+  xrope *x, *y;
+  char buf[1];
+
+  if (xr_len(rope) <= i) {
+    return NULL;
+  }
+
+  buf[0] = c;
+
+  x = xr_sub(rope, 0, i);
+  y = xr_new_volatile(buf, 1);
+  rope = xr_cat(x, y);
+  XROPE_DECREF(x);
+  XROPE_DECREF(y);
+
+  x = rope;
+  y = xr_sub(rope, i + 1, xr_len(rope));
+  rope = xr_cat(x, y);
+  XROPE_DECREF(x);
+  XROPE_DECREF(y);
+
+  return rope;
 }
 
 void
 pic_str_set(pic_state *pic, pic_str *str, size_t i, char c)
 {
-  UNUSED(pic);
+  xrope *x;
 
-  str->str[i] = c;
+  x = xr_put(str->rope, i, c);
+  if (x == NULL) {
+    pic_errorf(pic, "index out of range %d", i);
+  }
+  XROPE_DECREF(str->rope);
+  str->rope = x;
 }
 
 pic_str *
 pic_strcat(pic_state *pic, pic_str *a, pic_str *b)
 {
-  size_t len;
-  char *buf;
-
-  len = a->len + b->len;
-  buf = pic_alloc(pic, len + 1);
-
-  memcpy(buf, a->str, a->len);
-  memcpy(buf + a->len, b->str, b->len);
-  buf[len] = '\0';
-
-  return pic_str_new(pic, buf, len);
+  return str_new_rope(pic, xr_cat(a->rope, b->rope));
 }
 
 pic_str *
 pic_substr(pic_state *pic, pic_str *str, size_t s, size_t e)
 {
-  size_t len;
-  char *buf;
-
-  len = e - s;
-  buf = pic_alloc(pic, len + 1);
-
-  memcpy(buf, str->str + s, len);
-  buf[len] = '\0';
-
-  return pic_str_new(pic, buf, len);
+  return str_new_rope(pic, xr_sub(str->rope, s, e));
 }
 
 int
 pic_strcmp(pic_str *str1, pic_str *str2)
 {
-  return strcmp(str1->str, str2->str);
+  return strcmp(xr_cstr(str1->rope), xr_cstr(str2->rope));
+}
+
+const char *
+pic_str_cstr(pic_str *str)
+{
+  return xr_cstr(str->rope);
 }
 
 pic_value
diff --git a/src/symbol.c b/src/symbol.c
index a4159b32..7076f3ec 100644
--- a/src/symbol.c
+++ b/src/symbol.c
@@ -93,7 +93,7 @@ pic_symbol_string_to_symbol(pic_state *pic)
     pic_error(pic, "string->symbol: expected string");
   }
 
-  return pic_symbol_value(pic_intern_cstr(pic, pic_str_ptr(v)->str));
+  return pic_symbol_value(pic_intern_cstr(pic, pic_str_cstr(pic_str_ptr(v))));
 }
 
 void
diff --git a/src/vm.c b/src/vm.c
index 970f48a6..b483841e 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -188,15 +188,15 @@ pic_get_args(pic_state *pic, const char *format, ...)
     }
     case 'z': {
       pic_value str;
-      char **cstr;
+      const char **cstr;
 
-      cstr = va_arg(ap, char **);
+      cstr = va_arg(ap, const char **);
       if (i < argc) {
         str = GET_OPERAND(pic,i);
         if (! pic_str_p(str)) {
           pic_error(pic, "pic_get_args: expected string");
         }
-        *cstr = pic_str_ptr(str)->str;
+        *cstr = pic_str_cstr(pic_str_ptr(str));
         i++;
       }
       break;
diff --git a/src/write.c b/src/write.c
index 3adcda38..0a7bcafe 100644
--- a/src/write.c
+++ b/src/write.c
@@ -155,11 +155,11 @@ static void
 write_str(pic_state *pic, struct pic_string *str, xFILE *file)
 {
   size_t i;
-  const char *cstr = str->str;
+  const char *cstr = pic_str_cstr(str);
 
   UNUSED(pic);
 
-  for (i = 0; i < str->len; ++i) {
+  for (i = 0; i < pic_strlen(str); ++i) {
     if (cstr[i] == '"' || cstr[i] == '\\') {
       xfputc('\\', file);
     }