Abstract
In this paper an effort has been undertaken to view the family Mycobacteriaceae from the standpoint of general biology, evolution, and ecology at the microscopic level. By their range of tolerances for a variety of adverse environments, by their association with geographic and unknown soil and climatic factors, by their ability to survive and reproduce in sparse media, the atypical mycobacteria seem to possess characteristics of free-living organisms, whose contact with and importance to man results from overlapping niches, well-defined in the case of most mycobacteria, but less so in the case of Mycobacterium kansasii. In contrast, the human tubercle bacillus appears as a highly specialized organism with its niche restricted to mammalian tissue.