The antibiotic susceptibilities of 29 isolates of Plesiomonas shigelloides were studied with 24 antibiotics and antibiotic-inhibitor combinations. Results indicated that all isolates were susceptible to the cephalosporins, penicillins combined with a beta-lactamase inhibitor, aztreonam, and ciprofloxacin. Most isolates were resistant to the penicillins, possibly via production of a penicillinase.