Broth microdilution MICs were determined for 258 clinical isolates of Mycobacterium fortuitum (3 biovariants) and M. chelonae (2 subspecies) with amikacin, tobramycin, cefoxitin, doxycycline, erythromycin, and sulfamethoxazole-trimethoprim and with several new beta-lactams and aminoglycosides and ciprofloxacin. Variations in susceptibility by and within species subgroups confirm the need for susceptibility testing against clinically important strains.