The genome of the anaerobic thermophilic eubacterium Caldocellum saccharolyticum was analyzed by pulsed field gel electrophoresis. The restriction endonucleases NheI and SstII cleaved the genome into 23 and 21 fragments, respectively, and the sums of the fragments gave a genome size of approximately 2.78 Mbp. Hybridization of probes for several cellulolytic and hemicellulolytic genes, as well as a 16S ribosomal RNA gene, gave complex signals on PFGE-separated restriction fragments. Mapping of the genome by use of linking clone hybridizations had been initiated.