SUMMARY An Rdaiisonian classification of mycobacteria has been done by using 94 characters. Fifty-nine strains of slowly growing mycobacteria. were classified into five groups: (1) Mycobacterium tuberculosis and M. bovis (2) M. kansasii; (3) M. avium, 16 strains of non-photochromogens from human sources, 6 strains of non-photochromogenic mycobacteria from soil sources, and scotochromogens from human sources (M. aquae) ; (4) 2 strains of non-photochromogens from human sources; (5) 1 strain of non-photo- chromogen from human source. The third group seemed to consist of three subgroups: (3, i) non- chromogens from soil sources; (3, ii) Mycobacterium avium and some non-photochromogens, which were inseparable from M, avium; (3, iii) some non-photochromogens from human sources resembling M. avium (but separable from it) and scotochromogens from human sources. Slowly growing non-photochromogenic mycobacteria from soil sources (subgroup 3, i) were considered to form a new species, M. terrae. A descrip- tion of this species is given. Seventy-eight strains of rapidly growing mycobacteria were classified into seven groups : (6) 6 strains of miscellaneous species, Mycobacterium marinum, M. balnei, M. platypoecilus, M. ranae and M. piscium; (7) M. thermoresistibile (sp.nov.); (8) M. phlei; (9) M. aurum (sp.nov.); (10) M. fortuitum and group IV rapid growers; (11) 1M. parafortuitum; (12) M. smegmatis. These groups seemed to form independent species. Myco- bacterium thermoresistibile is a new species capable of growing at 52'. Mycobacterium aurum is a new species consisting of rapidly growing scoto- chromogenic mycobacteria with urease, nicotinamidase and pyrazina- midase and some strains also with acetarnidase and allantoinase.