Greater than 90% of 131 strains of Aeromonas species were susceptible to the aminoglycosides, ureidopenicillins, extended-spectrum cephalosporins, aztreonam, quinolones, tetracycline, and chloramphenicol, and all were uniformly resistant to ampicillin. Except for amoxicillin-clavulanate, sulfonamide, trimethoprim, and trimethoprim-sulfamethoxazole, there was good correlation between the results obtained by the agar dilution and disk diffusion techniques.