16S rRNA gene sequences of three previously described amino-acid-fermenting anaerobes, Selenomonas acidaminovorans, 'Selenomonas acidaminophila' and Eubacterium acidaminophilum, were determined. All three were found to cluster within the Clostridium and related genera of the subphylum of the Gram-positive bacteria. The thermophile, S. acidaminovorans, formed an individual line of descent and was equidistantly placed between Dethiosulfovibrio peptidovorans and Anaerobaculum thermoterrenum (similarity of 85%), both of which also form single lines of descent. 'S. acidaminophila' was related to Clostridium quercicolum, a member of cluster IX, with a similarity of 90%, whereas E. acidaminophilum was closely related to Clostridium litorale (similarity of 96%) as a member of cluster XI. Based on the phylogenetic data presented in this report and the phenotypic descriptions of these bacteria published previously, it is recommended that S. acidaminovorans be transferred to a new genus, Thermanaerovibrio gen. nov., as Thermanaerovibrio acidaminovorans comb. nov. and 'Selenomonas acidaminophila' be transferred to a new genus, Anaeromusa gen. nov., as Anaeromusa acidaminophila comb. nov. Though the transfer of E. acidaminophilum to a new taxon is justified, this is not recommended until the taxonomic status of all the members of cluster XI has been reviewed.