Among the several thousand bifidobacteria of our collection, 244 strains from the feces of rabbits and suckling pigs, the rumen of cattle, the feces of breast- and bottle-fed infants, the feces of calves, and from sewage are recognized by means of deoxyribonucleic acid (DNA)-DNA hybridization (competition filter method) as belonging to four new, distinct DNA homology groups. Twenty reference DNA preparations from the type strains of the currently known species or from reference strains of the homology groups of Bifidobacterium were used for comparison. The phenotypic traits which distinguish these four groups from previously described species of the genus Bifidobacterium include gross morphology, fermentation characteristics, guanine plus cytosine content of the DNA, the interpeptide bridge of the cell-wall peptidoglycan, and the transaldolase and 6-phosphogluconate dehydrogenase isozyme pattern (by starch gel electrophoresis). The four groups are named and described as new species of the genus Bifidobacterium: B. cuniculi, B. choerinum, B. boum, and B. pseudocatenulatum. The type strains of these species are RA93 (=ATCC 27916), SU806 (=ATCC 27686), RU917 (=ATCC 27917), and B1279 (=ATCC 27919), respectively.