Mycobacterium avium exhibits two colonial forms, transparent and opaque, on 7H10 agar. The transparent variant of M. avium B2900 undergoes a spontaneous transition to the opaque form at the rate of 4.7 x 10(-5) to 3.5 x 10(-4) per cell per generation. The two variants differ in cell morphology and mass; the opaque cells are two to three times as large as the transparent in terms of dry weight, total protein, deoxyribonucleic acid, or carbohydrate. A search for auxotrophic mutants resulted in the induction, by nitrosoguanidine or ultraviolet irradiation, of conditional, salt-sensitive mutants. One such mutant requires methionine and tryptophan supplementation for growth in a medium containing a buffer at high molarity. The clinical and physiological significance of these findings is discussed.