A numerical analysis of 369 strains received as members of the genus Mycobacterium, Rhodococcus (formerly Gordona) , Nocardia, or Corynebacterium was carried out using 88 characters. Of the strains tested, 345 mycobacteria, rhodococci, and nocardiae formed four distinct clusters at a similarity level of 85%. The first cluster was composed of slowly growing mycobacteria; the second cluster, of strains of Mycobacterium flavescens and M. thermoresistibile; the third cluster, of strains of rapidly growing mycobacteria; and the fourth cluster, of strains of Rhodococcus and Nocardia. Strains of three species of rapidly growing mycobacteria, M. agri, M. smegmatis, and M. vaccae, were located outside of these clusters. The fourth cluster could be divided into three subclusters, one consisting of rhodococci plus some strains of Nocardia asteroides, a second consisting of the remaining strains of Nocardia, and a third consisting of strains of Rhodococcus aurantiacus (Gordona aurantiaca) and Jensenia canicruria.Mycobacterium, Rhodococcus (formerly Gordona), and Nocardia are closely related taxa belonging to the order Actinomycetales (21).In 1966, Tsukamura (13) reported that mycobacteria can be divided into two subgroups, which roughly correspond to slowly growing and rapidly growing mycobacteria, by numerical classification. These two subgroups differed from each other not only in growth rate, but also in several biological characters. Subsequently, Tsukamura and Mizuno (14, 24) proposed the subgeneric names Mycobacterium and Mycomycobacterium for these subgroups. Later, after having compared mycobacteria and nocardiae, Tsukamura (17) recommended that the subgenera Myco bacterium (slowly growing mycobacteria) and Mycomycobacterium (rapidly growing mycobacteria) be elevated to genus status, i.e., taxa equivalent in status to the genus Nocardia.In 1971, Tsukamura (18) proposed a new genus, Gordona, for slightly acid-fast organisms considered to be intermediate between Mycobacterium and Nocardia and placed it in the family Actinomycetaceae, to which the genus Nocardia belongs. It was later shown by Tsukamura that the genus Gordona contained strain ATCC 13808 of Rhodococcus rhodochrous (19), and it was stated that the generic name Gordona should be changed to Rhodococcus (20), as this is the first name used for an organism belonging to the genus proposed. Independently, Goodfellow (2) observed that "Mycobacterium" rhodochrous differs from nocardiae, and Goodfellow et al. (4) reported that it also differs from rapidly growing mycobacteria.The finding that "M. " rhodochrous differs from both Mycobacterium and Nocardia was conf m e d by an international cooperative study (5). Goodfellow and Alderson (3) also proposed the name Rhodococcus for the "M. " rhodochrous complex, in which they included the genus Gordona. These authors also included several new species in the genus Rhodococcus.The taxonomic relationships of the mycobacteria, rhodococci, and nocardiae were studied by Tsukamura (21), who included hypothetical median organi...