Eleven hundred and twenty-eight strains of anaerobic diphtheroids from human skin could be divided into two groups according to their colonial morphology on casein-yeast-lactate-glucose agar and their susceptibility to lysis by bacteriophages. Members of group I were sensitive and those of group I1 resistant to the action of phage. Biochemical tests confirmed this division among 200 strains, and permitted a subdivision of group I1 into a proteolytic and a non-proteolytic subgroup-IIA and IIB respectively. A simple scheme, based on colonial morphology, phage susceptibility, the production of indole, nitrate reduction, DNAase, gelatinase and caseinase formation, haemolysis, and the acidification of sucrose, sorbitol, and trehalose, could be used for the classification of these organisms.