In vitro testing of the activity of 15 antibacterial agents against 522 clinical isolates of Aeromonas species demonstrated some species-associated trends. Amoxicillin plus clavulanic acid was effective against approximately 45% of Aeromonas caviae and Aeromonas hydrophila, but all Aeromonas sobria isolates were resistant. Aztreonam, piperacillin and mezlocillin were highly active against all the strains of Aeromonas tested. Ticarcillin was equally effective against Aeromonas caviae and Aeromonas hydrophila, but more than 50% of Aeromonas sobria isolates were resistant. The latter species was more susceptible to cephalosporins than Aeromonas hydrophila and Aeromonas caviae. Chloramphenicol, tetracycline and trimethoprim-sulfamethoxazole were extremely active against all three Aeromonas species, likewise ofloxacin and ciprofloxacin. Aztreonam, third-generation cephalosporins, chloramphenicol and the quinolones can thus be considered for therapy of infections when Aeromonas is implicated.