diff --git a/docs/api/cyclone/foreign.md b/docs/api/cyclone/foreign.md
index ac188886..cd5182dd 100644
--- a/docs/api/cyclone/foreign.md
+++ b/docs/api/cyclone/foreign.md
@@ -70,7 +70,9 @@ Scheme | C
 `bignum` | `mp_int`
 `opaque` | `void *`
 `c-void` | `void`
+`thread-data` | `gc_thread_data *`
 
 Useful notes:
 - Use `opaque` if you need to handle any kind of C pointer.
 - Use `string` to handle C `const char*` (`symbol` is strictly used to represent Scheme symbols).
+- `thread-data` is a special type used to pass the current thread's `gc_thread_data` instance to a C function. Objects of this type are passed implicitly when making a Scheme function call.
diff --git a/examples/ffi/sub-bignums.scm b/examples/ffi/sub-bignums.scm
index 87092c61..c3870add 100644
--- a/examples/ffi/sub-bignums.scm
+++ b/examples/ffi/sub-bignums.scm
@@ -1,17 +1,20 @@
 ;; A basic example of subtracting 2 bignums using the FFI.
 ;;
-;; This example is notable because we need to pass the
-;; current thread's data object to C so that we can pass
-;; it along to functions in the Cyclone runtime.
+;; This example also shows how to pass the current thread's 
+;; data object to C using the thread-data type specifier.
+;; The thread data object is often needed so it can be passed 
+;; along to functions in the Cyclone runtime.
 ;;
-(import (scheme base) (scheme write) (cyclone foreign) (srfi 18))
+(import (scheme base) (scheme write) (cyclone foreign))
 (include-c-header "sub-bignums.h")
 
-(c-define sub-big-nums bignum "sub_big_nums" opaque bignum bignum)
+;; Define a C function receiving thread data and two bignum arguments
+;; Note thread data is passed implicitly, so calls to sub-big-num do
+;; not need to pass the thread data argument from scheme code.
+(c-define sub-big-nums bignum "sub_big_nums" thread-data bignum bignum)
 
 (display
   (sub-big-nums 
-     (current-thread-data)
      999999999999999999999999
      222222222222222222222222))
 (newline)
diff --git a/libs/cyclone/foreign.sld b/libs/cyclone/foreign.sld
index 122c5b70..39275d59 100644
--- a/libs/cyclone/foreign.sld
+++ b/libs/cyclone/foreign.sld
@@ -119,6 +119,8 @@
              (string-append "opaque_ptr(" ,code ")"))
             ((c-void)
              "Cyc_VOID")
+            ((thread-data)
+             "data")
             (else
               (error "scm->c unable to convert scheme object of type " ,type)))))))
   
@@ -275,9 +277,12 @@
                        "(void *data, int argc, closure _, object k " 
                        (apply string-append 
                          (map
-                           (lambda (sym/unbox)
-                             (string-append ", object " (car sym/unbox)))
-                         arg-syms/unbox))
+                           (lambda (sym/unbox type)
+                             (if (eq? type 'thread-data)
+                                 ""
+                                 (string-append ", object " (car sym/unbox))))
+                         arg-syms/unbox
+                         arg-types))
                         ")"))
                (type-checks
                  (apply