In recent years there have been increasing numbers of reports on the emergence of disseminated disease due to mycobacteria other than Mycobacterium tuberculosis. Isolation of these organisms from normally sterile sites including blood, bone marrow, and cerebrospinal fluid have led to a renewed interest in the ecology and human acquisition from environmental sources such as water. The current epidemic of Acquired Immune Deficiency Syndrome (AIDS) as well as changing demographics will demand a hard look and greater respect for these opportunistic pathogens by microbiologists and physicians who in the past have only regarded them as contaminants or transient colonizers. Interest in the ecology as well as the medical aspects of mycobacteria are the subject of the review.