Summary Twenty five strains, which belong to Bifidobacterium animalis on the basis of their carbohydrate fermentation patterns, were isolated from human and calf feces. The DNAs of these strains were, however, completely homologous to the DNA of the type strain of Bifidobacterium longum . In this study, the phenotypic characteristics for differentiating B. longum from B. animalis were investigated. B. longum showed mostly very elongated and relatively thin cells with slightly irregular contours, while B. animalis consisted of relatively short rods with a slightly enlarged central portion. Strains of B. longum failed to grow in the presence of 4.2% NaCl at 37°C or 3.0% NaCl at 45 °C, and possessed N-acetyl-β-glucosaminidase, while B. animalis strains grew in either culture condition, possessed β-glucosidase and phosphoamidase, but not N-acetyl-β-glucosaminidase.