Restriction fragment length polymorphism (RFLP) types of Mycobacterium avium intracellulare and Mycobacterium kansasii isolated from patients with AIDS were examined. We demonstrate that one RFLP type is much more common than others which confirms previous findings. Carriage of individual RFLP types is constant over long periods of time. In addition, we document a disseminated infection in a patient with M. avium intracellulare of three RFLP types. © 1992 The British Society for the Study of Infection.