Mycobacterium avium, a pathogen of both animals and humans, is an acid-fast bacterium that is very drug-resistant and pleomorphic in colony and cellular morphology. By selective filtration, cells 1 μm long could be obtained. When placed in fresh medium, these small cells elongated to form filaments that aggregated during about a 40-hr incubation period. The filamentous cells divided rapidly for an additional 40 hr, with a doubling time of ∼6 hr. Fission ceased, and the resulting culture consisted of coccobacilli. The cell cycle would not proceed if the cells were starved for either fatty acid or ammonium ion. During elongation (the growth phase), protein, DNA, and triglycerides were synthesized exponentially. During the fission stage, the triglycerides were utilized and redistributed among other cellular constituents. It is proposed that the cell cycle offers a unique system by which to test drugs that may inhibit growth of, or be bactericidal for, M. avium.