Strains of the species Mycobacterium simiae give a positive niacin test. On the basis of their cultural and biochemical characteristics and by seroagglutination they can be classified into 2 subspecies. (1) The strains of serotype M. simiae 1 hydrolyze urea regularly and nicotinamide and pyrazinamide irregularly. They are photochromogenic after prolonged exposure to light. (2) The strains of serotype M. simiae 2 hydrolyze urea only. Three of 4 strains are scotochromogenic; the fourth has a pale pink pigment. Two of these strains possess alpha- and beta-esterase activity. The other two are negative in this test. Strains of M. habana are culturally and biochemically identical with the serotype M. simiae 1 and show the same serologic specificity as M. simiae 1. M. habana sera, after having been absorbed by M. simiae 1, retain a small amount of agglutinins specific for M. habana. We believe that the M. habana strains belong to the species M. simiae and are closely related to the serotype M. simiae 1.