By use of a murine model for Buruli ulcer, Mycobacterium ulcerans was found to be susceptible to rifampin, with the MIC being 0.5 to 1 micro g/ml. Three mutants were isolated after rifampin monotherapy. Two were resistant to rifampin at 8 micro g/ml, and one was resistant to rifampin at 32 micro g/ml. The mutants harbored Ser416Phe mutations and His420Tyr mutations in the rpoB gene, and these mutations have also been found to be responsible for rifampin resistance in the leprosy and tubercle bacilli. The results indicate that while rifampin may be active against M. ulcerans, it should never be used as monotherapy in humans.