Several studies have suggested that large bodies of water are a main source of infection with mycobacteria other than tuberculosis (MOTT). If this is correct, there should be a gradient in the infection rate with MOTT between mountainous and seaside areas. To test this hypothesis, we performed skin testing with tuberculin and sensitins in 19,470 Greek Armed Forces recruits. Initially, several MOTT sensitins were used, but when it became clear that the Mycobacterium scrofulaceum sensitin was the most appropriate, the study was continued with it alone in 17,403 recruits. Finally, in order to evaluate the geographical distribution of sensitivity to sensitins, we studied the results of 8,507 of these recruits living in or near their birthplace. They were divided into three geophysical areas: seaside 3,389 recruits; mountains 2,692 recruits; and inland plains 2,426 recruits. MOTT sensitivity rates were 4.1% in mountainous areas and 7.1% in seaside areas. All small Aegean islands had high MOTT rates (above 8%). In inland plains, high MOTT rates (above 8%) were observed among those living near big rivers. This geographical distribution of MOTT sensitivity supports the theory that large bodies of water are a main source of infection with MOTT.