Bacteria classified as extreme halophiles, in the genera Halobacterium and Halococcus, contain deoxyribonucleic acid (DNA) which displays two components in a CsCl equilibrium density gradient. The base composition of the major DNA component ranges from 66 to 68% guanine plus cytosine (GC), whereas that of the satellite DNA comprising some 11 to 36% of the total, is between 57 and 60% GC. Purification of the bacterial cells in a CsCl density gradient and other more conventional strain purification procedures both indicated that the presence of the satellite DNA component is not a result of mixed cultures.