The Mycobacterium species M. tuberculosis, M. avium-intracellulare, M. kansasii, M. marinum, M. scrofulaceum, M. fortuitum, M. terrae , and M. gordonae were analyzed for their susceptibility to rifampin. M. tuberculosis, M. kansasii , and M. marinum were susceptible to the antibiotic, and resistant populations developed as a result of the interplay of mutation and selection. The mutation rates (susceptibility → resistance) were calculated to be 4.9 × 10 −10 and 1.7 × 10 −9 mutations per bacterium per generation in, respectively, M. kansasii and M. marinum. M. fortuitum was found to be naturally resistant to the antibiotic, whereas the nature of resistance in the other species was unclear and is discussed.