Natural lysogeny could not be demonstrated in 34 stock cultures of representative saprophytic and pathogenic mycobacteria with no previous history of laboratory exposure to mycobacteriophage. It has been demonstrated that a number of these strains can be lysogenized by exposure to lytic or temperate mycobacteriophage under laboratory conditions. Phage-induced alterations in cultural or biochemical reactions were not detected among the lysogenized strains. Lysogenic cultures of Mycobacterium tuberculosis H37RV and M. fortuitum are virulent and remain lysogenic through animal passage.