Numerical analysis of the relationships among rapidly growing, scotochromogenic mycobacteria was carried out; a dendrogram was prepared by a single linkage method and the separation between species was tested. The following were distinct species: Mycobacterium alichiense; mycobacterium rhodesiae;Mycobacterium chubuense; Mycobacterium obuense; Mycobacterium gilvum; mycobacterium duvalii; mycobacterium flavescens; and mycobacterium vaccae. Mycobacterium tokaiense was reduced to a synonym of M. chubense. Mycobacterium parafortuitum, Mycobacterium aurum and Mycobacterium neoaurum were considered to be subspecies of M. parafortuitum or part of a M. parafortuitum complex. The separation between species was tested by comparing the extent of overlap of one species with that of another. When two species showed no such overlap,they were regarded as distinct. The results of the test of separation agreed well with taxonomic relationships shown by the dendrogram. Characters useful for differentiating the species were found.