A group of 20 alkaliphilic Bacillus strains in which all strains revealed the same unique comination of properties--obligate alkaliphily, oval spores distending the sporangium, and ornithine and aspartic acid instead of diaminopimelic acid in the cell wall--was examined. Most of the strains had been isolated by a five-step enrichment and isolation procedure. The G+C content was determined to span a range from 33.5 to 35.0 mol%. Unsaturated fatty acids amounted to 17 to 28% of the total cellular fatty acids. Through DNA-DNA hybridization experiments 11 strains could be grouped in one species. Low homology values with the type strains of validly published Bacillus species with similar G+C contents suggest that these strains belong to a hitherto undescribed species for which the name Bacillus cohnii is proposed. The type strain of the new species is strain RSH (= DSM 6307).