Mycobacterium celatum is a recently described slow-growing species. It was identified on the basis of genomic sequencing that differentiates three types. The present report describes two cases of Mycobacterium celatum type I infection in patients with AIDS. Both patients had CD4+ lymphocyte counts of <10/mm3, were receiving rifabutin prophylaxis, and had attended the same treatment units. The minimum inhibitory concentration of rifabutin for both strains was 8 mg/l, which may account for the failure of prophylaxis. As all type 1 strains have the same pulsedfield gel electrophoresis pattern, nosocomial transmission or acquisition from a common source could not be ruled out.