We report here the first available data on the genomic structure of an archaebacterial, extremely halophilic coccus. Thus, the physical map of a region of 257 kilobase-pairs (kbp) from the extremely halophilic aerobic archaebacterium Halococcus saccharolyticus ATCC 49257 has been constructed. This DNA fragment could be ascribed to the FI fraction of the chromosome. Long repeated sequences were not detected, but, on the other hand, some losses of fragments in the recombinants were found. These data suggest that only a minor part of the chromosome of H. saccharolyticus, which is not cccDNA, is accompanied by multiple rearrangements, while the other, which includes most of the chromosome, possesses more genetic stability. Our results indicate that there is a low degree of genomic variability in contrast to the high instability reported for Halobacterium salinarium.