diff --git a/lib/mt/CThreadRep.cpp b/lib/mt/CThreadRep.cpp
index 82e15992..ceff76c2 100644
--- a/lib/mt/CThreadRep.cpp
+++ b/lib/mt/CThreadRep.cpp
@@ -24,9 +24,6 @@
 
 #endif
 
-// FIXME -- temporary exception type
-class XThreadUnavailable { };
-
 //
 // CThreadRep
 //
diff --git a/lib/mt/XThread.h b/lib/mt/XThread.h
index 3a5e59e1..700c195b 100644
--- a/lib/mt/XThread.h
+++ b/lib/mt/XThread.h
@@ -27,6 +27,12 @@ must rethrow it if caught (by XThreadCancel, XThread, or ...).
 */
 class XThreadCancel : public XThread { };
 
+//! Thread creation exception
+/*!
+Thrown when a thread cannot be created.
+*/
+class XThreadUnavailable { };
+
 /*!
 \def RETHROW_XTHREAD
 Convenience macro to rethrow an XThread exception but ignore other