Strains of scotochromogenic mycobacteria were studied by using numerical taxonomy methods in an attempt to more clearly define Mycobacterium szulgai and to find tests useful in identifying the species. In this study all strains of M. szulgai were strong reducers of nitrate, were slow in hydrolyzing Tween 80, and gave a high semiquantitative catalase reaction. Results obtained indicate that the use of increased pigmentation after 1 h of light exposure at 25 C and that the use of arylsulfatase activity are of questionable diagnostic value in separating the species from other scotochrompgenic mycobacteria.