MPICH is probably the most widely used implemen- tation of MPI. Recent versions of MPICH optimize the performance of some collective communications by switching between different algorithms depending on whether the message size is greater or less than a given change-over point, which is currently hard- coded in MPICH, based on measurements on clusters with one CPU per node. We have used MPI bench- marks to find the optimum change-over points for different systems, and found that they can vary sig- nificantly for different networks and different numbers of processes per node. In some cases significant per- formance improvements can be obtained by enabling MPICH to be customized in this way, particularly on clusters with more than one CPU per node. KEY WORDS MPI benchmarks, parallel computer, network per- formance, collective communication.