Plesiomonas shigelloides was isolated from the stools of two patients with gastrointestinal malignancies and from one "healthy" patient with diarrhea. No other enteric pathogens were isolated. One patient was neutropenic after antineoplastic chemotherapy. The two cancer patients had crampy abdominal pain and severe diarrhea and required antibiotic therapy for resolution of symptoms. The third patient responded to symptomatic treatment. Patients with gastrointestinal malignancies may be more susceptible to gastroenteritis caused by P. shigelloides than normal individuals, and these infections may be more severe than those seen in normal individuals.