33 ground water samples from three drinking water treatment plants and 72 samples from domestic drinking water distribution systems were studied for the occurrence of mycobacteria. 86 out of these samples tested positive for mycobacteria with concentrations generally ranging between 10(2) and 10(3) cfu/l. In one distribution system up to 4.5 x 10(5) cfu/l were found. Species identified by biochemical reactions and by thin layer chromatography of mycolic acids included: Mycobacterium gordonae (most water samples), M. flavescens, M. kansasii, M. chelonae (domestic water systems) and M. fortuitum (drinking water treatment plant). Most isolates did not show patterns of biochemical reactivity attributable to any known mycobacterial species.