The virulence of Nocardia brasiliensis, N. asteroides, and N. caviae was investigated. The study was done by inoculation of these organisms into the footpad of white mice. The results show that N. brasiliensis has greater virulence than N. asteroides and N. caviae. These observations resemble clinical findings in which N. brasiliensis acts as an obligatory pathogenic agent, whereas N. asteroides usually behaves as an opportunistic pathogen. Nocardia caviae, which is very similar to N. asteroides from the taxonomic standpoint, is only rarely a causative agent for mycetoma.