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 4 distinct clusters at a similarity level of 85%. The 1st cluster was composed of slowly growing mycobacteria; the 2nd cluster, of strains of M. flavescens and M. thermoresistibile; the 3rd cluster, of strains of rapidly growing mycobacteria; and the 4th cluster, of strains of Rhodococcus and Nocardia. Strains of 3 spp. of rapidly growing mycobacteria, M. agri, M. smegmatis and M. vaccae, were located outside of these clusters. The 4th cluster could be divided into 3 subclusters, 1 consisting of rhodococci plus some strains of N. asteroides, a 2nd consisting of the remaining strains of Nocardia and a 3rd consisting of strains of R. aurantiacus (Gordona aurantiaca) and Jensenia canicruria.