Complete sequences of 16S rRNA encoding genes from extreme halophiles Halobacterium saccharovorum, Halobacterium lacusprofundi, and Halobacterium distributum were determined. The polar lipids, particularly the glycolipids, of these and 20 other isolates were also analyzed. Based on both genetic and chemotaxonomic data, the following two novel genera are proposed: Halorubrobacterium (comprising Halorubrobacterium saccharovorum, Halorubrobacterium sodomense, Halorubrobacterium lacusprofundi, Halorubrobacterium distributum, and Halorubrobacterium coriensis) and Natrialba (comprising Natrialba asiatica). Evidence for a third new genus is also presented.Halobacteria are a group of microorganisms forming part of the domain Archaea, and require high salt concentrations for growth (11). For many years the taxonomy of halobacteria has been confused, partly the result of a classification system not based on rRNA sequence data, and partly due to many of the deposited strains retaining their original classification in the genus Halobacterium even after the establishment of the genera Haloarcula and Haloferax (36). Adding to this confusion has been the use of misidentified or mislabeled cultures in taxonomic studies, giving rise to conflicting data, e.g., Hb. saccharovorum (32,34) and Hb. trapanicum (34), and delaying a clear definition of genera. The ramifications of this situation are directly affecting progress in the area and there is an urgent need for reform (32). This is becoming more pressing as new halobacteria are discovered (22, 24, 2 7, 28) .Currently, the family Halobacteriaceae contains six genera of extremely halophilic archaea: Halobacterium, Haloarcula, Haloferax, Halococcus, and the two alkaliphilic genera, Natronobacterium and Natronococcus (11). These divisions are * Address reprint requests to:399 Noda,