Comparative 16S rRNA sequence analysis has demonstrated that the genus Bacillus consists of at least five phyletic lines. rRNA group 3 bacilli of Ash, Farrow, Wallbanks and Collins (1991) comprising Bacillus polymyxa and close relatives is phylogenetically so removed from Bacillus subtilis, the type species of the genus and other aerobic, endospore-forming bacilli that they warrant reclassification in a new genus Paenibacillus. The genus Paenibacillus can be readily distinguished from other Bacillus groups using a battery of phenotypic characters and a highly specific gene probe based on 16S rRNA.