Apple Xserve-powered computer clusters have two of the top twenty positions in the Top 500 Supercomputer lists.

Viginia Tech's 'System X' Xserve-based cluster has fallen from 14th place in June to 20th place in the most recent independent lists. System X exploits 1,200 dual 2.3GHz G5 Xserves.

In fifteenth place sits the Mach 5 Xserve-based supercomputer, which is used by both the US Army and NASA for research and development.

For the third time in the bi-annual lists, IBM held its lead with the BlueGene/L system at the US Department of Energy's (DOE's) Lawrence Livermore National Laboratories.

IBM processors power five of the world's top ten supercomputers, though two-thirds of the Top 500 machines are powered by Intel processors.

Also in the top 500 list (at number 303) is the University of California's Dawson, an Xserve-based cluster. This is followed at number 308 by Xseed, which is installed at Bowie University in the US.

The UK now has 41 systems on the top 500 list.