Minimal inhibitory concentrations and minimal bactericidal concentrations of colistin (polymyxin E) were determined for the type strains of fifteen mycobacterial species. Colistin was found to be active against pathogenic species Mycobacterium xenopi, M. intracellulare, M. tuberculosis, M. fortuitum and also against the rapidly growing, non-pathogenic species M. phlei and M. smegmatis. The discriminatory potential of susceptibility to colistin as a test was investigated on 25 strains of the M. fortuitum/M. chelonei complex, and also on 11 strains of the M. avium/M. intracellulare complex. The experimental data indicated the potential of colistin susceptibility testing for discriminating M. fortuitum from M. chelonei.