diff -ur pl-5.6.25.orig/packages/clib/testsocket.pl pl-5.6.25/packages/clib/testsocket.pl
--- pl-5.6.25.orig/packages/clib/testsocket.pl	2004-09-22 02:28:16.000000000 +1200
+++ pl-5.6.25/packages/clib/testsocket.pl	2007-02-24 08:55:29.000000000 +1300
@@ -35,6 +35,12 @@
 	    client/1			% +Address
 	  ]).
 
+:- (   current_prolog_flag(threads, true)
+   ->  true
+   ;   format('Disabling tcp_test on single-threaded engine~n~n', []),
+       halt
+   ).
+
 :- asserta(user:file_search_path(foreign, '.')).
 
 :- use_module(socket).
diff -ur pl-5.6.25.orig/packages/semweb/litmap_test.pl pl-5.6.25/packages/semweb/litmap_test.pl
--- pl-5.6.25.orig/packages/semweb/litmap_test.pl	2006-03-07 02:17:30.000000000 +1300
+++ pl-5.6.25/packages/semweb/litmap_test.pl	2007-02-24 08:55:29.000000000 +1300
@@ -3,6 +3,13 @@
 	    test/1,
 	    test/2
 	  ]).
+
+:- (   current_prolog_flag(threads, true)
+   ->  true
+   ;   format('Disabling literal map test on single-threaded engine~n~n', []),
+       halt
+   ).
+
 :- use_module(library(debug)).
 :- use_module(library(debug)).
 :- asserta(user:file_search_path(foreign, '../sgml')).
diff -ur pl-5.6.25.orig/packages/semweb/rdf_db_test.pl pl-5.6.25/packages/semweb/rdf_db_test.pl
--- pl-5.6.25.orig/packages/semweb/rdf_db_test.pl	2006-11-10 22:44:56.000000000 +1300
+++ pl-5.6.25/packages/semweb/rdf_db_test.pl	2007-02-24 08:55:29.000000000 +1300
@@ -29,6 +29,12 @@
     the GNU General Public License.
 */
 
+:- (   current_prolog_flag(threads, true)
+   ->  true
+   ;   format('Disabling RDF_DB test suite on single-threaded engine~n~n', []),
+       halt
+   ).
+
 :- asserta(file_search_path(foreign, '../sgml')).
 :- asserta(file_search_path(library, '../sgml')).
 :- asserta(file_search_path(library, '../sgml/RDF')).
Only in pl-5.6.25/packages/semweb: rdf_db_test.pl.orig
diff -ur pl-5.6.25.orig/packages/ssl/ssl_test.pl pl-5.6.25/packages/ssl/ssl_test.pl
--- pl-5.6.25.orig/packages/ssl/ssl_test.pl	2006-06-21 19:51:59.000000000 +1200
+++ pl-5.6.25/packages/ssl/ssl_test.pl	2007-02-24 08:55:29.000000000 +1300
@@ -29,6 +29,12 @@
     the GNU General Public License.
 */
 
+:- (   current_prolog_flag(threads, true)
+   ->  true
+   ;   format('Disabling SSL test on single-threaded engine~n~n', []),
+       halt
+   ).
+
 :- asserta(user:file_search_path(foreign, '.')).
 
 :- use_module(ssl).
diff -ur pl-5.6.25.orig/packages/zlib/ztest.pl pl-5.6.25/packages/zlib/ztest.pl
--- pl-5.6.25.orig/packages/zlib/ztest.pl	2006-12-06 01:19:44.000000000 +1300
+++ pl-5.6.25/packages/zlib/ztest.pl	2007-02-24 09:05:57.000000000 +1300
@@ -56,7 +56,7 @@
 
 %	zstream: test compressed stream flushing and processing
 
-test(zstream) :-
+test(zstream, [condition(current_prolog_flag(threads, true))]) :-
 	server(Port),
 	debug(server, 'Server at ~w~n', [Port]),
 	client(Port),