MICs of 22 antimicrobial agents for 60 strains of three Aeromonas species were determined by a microdilution method. The newer cephalosporins such as moxalactam, cefotaxime, and cefoperazone, the aminoglycosides, and chloramphenicol, tetracycline, nitrofurantoin, and trimethoprim-sulfamethoxazole inhibited most of the strains studied. Within the genus, A. hydrophila was more resistant than either A. caviae or A. sobria to the antibiotics tested.