The almost complete sequence of the 16S rRNA gene of the Gram-positive polysporogenic bacterium Anaerobacter polyendosporus was determined. This allowed phylogenetic analysis of A. polyendosporus by comparing sequences of the 16S rRNA gene of this bacterium to similar genes of other Gram-positive bacteria. It was shown that this polysporogenic bacterium belongs to the Clostridium cluster I, subcluster A. Phylogenetically, A. polyendosporus is distantly related to another polysporogenic, but non-cultivatable, bacterium, 'Metabacterium polyspora' and can be satisfactorily clustered within the saccharolytic clostridia with a low DNA G+C content grouped in subcluster A. A. polyendosporus was most closely related to Clostridium intestinale (94.8% identity of 16S rRNA genes) and Clostridium fallax (93.1%). Like other members of the Clostridium cluster I, subcluster A, A. polyendosporus possesses such common phenotypic features as a Gram-positive cell wall structure, anaerobiosis, derivation of energy from carbohydrate fermentation yielding butyric acid among other organic acids and the capacity for endogenous spore-formation. However, the scale of evolutionary change in the 16S rRNA gene between A. polyendosporus and phylogenetically related Clostridium species does not correspond to the profound changes in the phenotype of A. polyendosporus. Distinctive phenotypic features of the latter are large cell size, polysporogenesis (up to seven spores per cell), alternative modes of development and an unusual membrane ultrastructure.