Environmental mycobacteria are a common cause of human infections. Recently, contaminated domestic water supplies have been suggested as a potential environmental source of several mycobacterial diseases. Since many of these mycobacterial species replicate best intracellularly, environmental hosts have been sought. In the present study, we examined the interaction of Mycobacterium avium with a potential protozoan host, the water-borne amoeba Acanthamoeba castellanii. We found that M. avium enters and replicates in A. castellanii. In addition, similar to that shown for mycobacteria within macrophages, M. avium inhibits lysosomal fusion and replicates in vacuoles that are tightly juxtaposed to the bacterial surfaces within amoebae. In order to determine whether growth of M. avium in amoebae plays a role in human infections, we tested the effects of this growth condition on virulence. We found that growth of M. avium in amoebae enhances both entry and intracellular replication compared to growth of bacteria in broth. Furthermore, amoeba-grown M. avium was also more virulent in the beige mouse model of infection. These data suggest a role for protozoa present in water environments as hosts for pathogenic mycobacteria, particularly M. avium.