Summary: Strains of ‘Lactobacillus’ bifidus were found to vary in morphology from large globules to more slender septate rods and filaments, often very profusely branched. Adaptation of anaerobic strains to growth under aerobic conditions is usually accompanied by a diminution in branching. The organism has little or no morphological resemblance to a true Lactobacillus species, but strongly resembles certain strains of Corynebacterium.