An 83-year-old male presented with chronic diarrhea and protein malnutrition associated with Plesiomonas shigelloides overgrowth in the small intestine. This overgrowth was related to achlorhydria and small-bowel diverticula. Tests for heat-labile and heat-stable enterotoxin were negative on the P. shigelloides isolated from both the small bowel aspirate (3 x 10(8) bacteria per ml) and the patient's stool. The patient responded satisfactorily to tetracycline, to which this unusual isolate was susceptible in vitro. The ability to correctly identify and determine the antimicrobial susceptibility of P. shigelloides in the setting of small-bowel overgrowth will help determine appropriate therapy.