This paper describes some preliminary results about a workload characterization study at a national supercomputer center. This study is part of a larger project to develop an analytic methodology for the construction of arepresentative benchmark set. Our study reports on 292,254 user jobs during the period from June 1992 to July 1993 and represents about one-half of the wall clock time available on the four-processor CRAY Y-MP at the National Center for Supercomputing Applications during this time. The performance statistics we gathered in some sensedefine the workload because of the large fraction of the available time that was recorded. These statistics will allow us to reverse engineer a set of benchmarks that analytically represents the workload at this site.