To evaluate the possibility of separate pulmonary infections in human beings by different strains of Mycobacterium tuberculosis, a search for multiple phage types within a single host was under-taken. Culture isolates from 2 or more distinct anatomic sites of infection in the same patient were obtained from 87 persons. In 3 subjects, 2 distinct phage types were found. The possible explanations for 2 types in the same patient and the epidemiologic implications are discussed.