CLUSTER2000 recognizes clusters in space-time in an earthquake catalog. It is intended for use in removing aftershocks or "declustering" the catalog. The methods used are described in "Second-Order Moment of Central California Seismicity, 1969-1982" (P. Reasenberg, JGR v.90., pp.5479-5495, 1985). The current version (CLUSTER2000x) reads catalogs in a variety of standard formats. However, it is not fully Y2000 compatible. While it reads the Y2K formats for HYPOINVERSE and HYPO71, CLUSTER2000x requires all data to belong to one century.