diff --git a/include/cyclone/runtime.h b/include/cyclone/runtime.h
index d7ec7d42..c08d4fad 100644
--- a/include/cyclone/runtime.h
+++ b/include/cyclone/runtime.h
@@ -233,7 +233,6 @@ extern const object primitive_Cyc_91cvar_127;
 extern const object primitive_Cyc_91has_91cycle_127;
 extern const object primitive_Cyc_91spawn_91thread_67;
 extern const object primitive_Cyc_91end_91thread_67;
-extern const object primitive_thread_91sleep_67;
 extern const object primitive_Cyc_91minor_91gc;
 extern const object primitive__87;
 extern const object primitive__91;
diff --git a/runtime.c b/runtime.c
index df5d476b..a8c05a7b 100644
--- a/runtime.c
+++ b/runtime.c
@@ -1783,9 +1783,6 @@ void _Cyc_91spawn_91thread_67(void *data, object cont, object args) {
 void _Cyc_91end_91thread_67(void *data, object cont, object args) {
     Cyc_end_thread((gc_thread_data *)data);
     return_closcall1(data, cont, boolean_f); }
-void _thread_91sleep_67(void *data, object cont, object args) {
-    Cyc_check_num_args(data, "thread-sleep!", 1, args);
-    return_closcall1(data, cont, Cyc_thread_sleep(data, car(args))); }
 void _Cyc_91minor_91gc_primitive(void *data, object cont, object args){
     Cyc_trigger_minor_gc(data, cont); }
 void __87(void *data, object cont, object args) {
@@ -2526,7 +2523,6 @@ static primitive_type Cyc_91cvar_127_primitive = {{0}, primitive_tag, "Cyc-cvar?
 static primitive_type Cyc_91has_91cycle_127_primitive = {{0}, primitive_tag, "Cyc-has-cycle?", &_Cyc_91has_91cycle_127};
 static primitive_type Cyc_91spawn_91thread_67_primitive = {{0}, primitive_tag, "Cyc-spawn-thread!", &_Cyc_91spawn_91thread_67};
 static primitive_type Cyc_91end_91thread_67_primitive = {{0}, primitive_tag, "Cyc-end-thread!", &_Cyc_91end_91thread_67};
-static primitive_type thread_91sleep_67_primitive = {{0}, primitive_tag, "thread-sleep!", &_thread_91sleep_67};
 static primitive_type Cyc_91minor_91gc_primitive = {{0}, primitive_tag, "Cyc-minor-gc", &_Cyc_91minor_91gc_primitive};
 static primitive_type _87_primitive = {{0}, primitive_tag, "+", &__87};
 static primitive_type _91_primitive = {{0}, primitive_tag, "-", &__91};
@@ -2651,7 +2647,6 @@ const object primitive_Cyc_91cvar_127 = &Cyc_91cvar_127_primitive;
 const object primitive_Cyc_91has_91cycle_127 = &Cyc_91has_91cycle_127_primitive;
 const object primitive_Cyc_91spawn_91thread_67 = &Cyc_91spawn_91thread_67_primitive;
 const object primitive_Cyc_91end_91thread_67 = &Cyc_91end_91thread_67_primitive;
-const object primitive_thread_91sleep_67 = &thread_91sleep_67_primitive;
 const object primitive_Cyc_91minor_91gc = &Cyc_91minor_91gc_primitive;
 const object primitive__87 = &_87_primitive;
 const object primitive__91 = &_91_primitive;
diff --git a/scheme/cyclone/cgen.sld b/scheme/cyclone/cgen.sld
index c880ed31..26ce0363 100644
--- a/scheme/cyclone/cgen.sld
+++ b/scheme/cyclone/cgen.sld
@@ -455,7 +455,6 @@
      ((eq? p 'Cyc-has-cycle?)        "Cyc_has_cycle")
      ((eq? p 'Cyc-spawn-thread!)     "Cyc_spawn_thread")
      ((eq? p 'Cyc-end-thread!)       "Cyc_end_thread")
-     ((eq? p 'thread-sleep!)         "Cyc_thread_sleep")
      ((eq? p 'Cyc-stdout)            "Cyc_stdout")
      ((eq? p 'Cyc-stdin)             "Cyc_stdin")
      ((eq? p 'Cyc-stderr)            "Cyc_stderr")
@@ -593,7 +592,6 @@
     Cyc-default-exception-handler
     Cyc-current-exception-handler
     Cyc-end-thread!
-    thread-sleep!
     open-input-file
     open-output-file
     close-port
diff --git a/scheme/cyclone/transforms.sld b/scheme/cyclone/transforms.sld
index 2eea5ea0..39f7da53 100644
--- a/scheme/cyclone/transforms.sld
+++ b/scheme/cyclone/transforms.sld
@@ -454,7 +454,6 @@
      Cyc-has-cycle?
      Cyc-spawn-thread!
      Cyc-end-thread!
-     thread-sleep!
      Cyc-minor-gc
      Cyc-stdout
      Cyc-stdin
@@ -565,7 +564,6 @@
                   Cyc-cvar?
                   Cyc-spawn-thread!
                   Cyc-end-thread!
-                  thread-sleep!
                   Cyc-minor-gc
                   apply
                   %halt
diff --git a/scheme/eval.sld b/scheme/eval.sld
index 053336e4..e7bec766 100644
--- a/scheme/eval.sld
+++ b/scheme/eval.sld
@@ -139,7 +139,6 @@
       (list 'Cyc-has-cycle? Cyc-has-cycle?)
       (list 'Cyc-spawn-thread! Cyc-spawn-thread!)
       (list 'Cyc-end-thread! Cyc-end-thread!)
-      (list 'thread-sleep! thread-sleep!)
       (list 'Cyc-default-exception-handler Cyc-default-exception-handler)
       (list 'Cyc-current-exception-handler Cyc-current-exception-handler)
       (list '+ +)
diff --git a/srfi/18.sld b/srfi/18.sld
index 8a5234a5..26a2b93b 100644
--- a/srfi/18.sld
+++ b/srfi/18.sld
@@ -1,12 +1,14 @@
 (define-library (srfi 18)
   (import (scheme base))
   (export
+    ;; TODO: current-thread
     thread?
     make-thread
     thread-name
     thread-specific
     thread-specific-set!
     thread-start!
+    thread-sleep!
     thread-yield!
 ;    thread-terminate!
     ; For now, these are built-ins. No need for them here: make-mutex mutex-lock! mutex-unlock!
@@ -50,6 +52,10 @@
 ;    (define (thread-terminate!) (Cyc-end-thread!))
     ;; TODO: thread-join!
 
+    (define-c thread-sleep!
+      "(void *data, int argc, closure _, object k, object timeout)"
+      " return_closcall1(data, k, Cyc_thread_sleep(data, timeout)); ")
+
     ;; Take a single object and if it is on the stack, return a copy
     ;; of it that is allocated on the heap. NOTE the original object
     ;; will still live on the stack, and will eventually be moved