The in vitro susceptibility of 39 strains of M. fortuitum to 14 beta-lactams and 11 aminoglycosides was studied using disc diffusion techniques on Mueller-Hinton agar plates. Amikacin, lividomycin, neomycin and kanamycin inhibited clearly all the strains, while penicillins, cephalosporins and streptomycin were inactive. Other tested aminoglycosides poorly inhibited the strains.