Thirty-one persons nationwide from whom Plesiomonas shigelloides was isolated in large numbers from stool in 1984 were compared with 62 matched control subjects. Infection with P. shigelloides was strongly associated with eating uncooked shellfish, usually raw oysters, in the 48 hours before the onset of illness (p less than 0.00001) and with foreign travel (p less than 0.00006), usually to Mexico. Most ill persons had self-limited diarrhea with blood and mucus in stool and other clinical findings that suggested enteroinvasiveness of infecting organisms. Two patients developed their illnesses after taking ampicillin for reasons unrelated to diarrhea; plesiomonads recovered from their stools were resistant to ampicillin. Seven persons with gastrointestinal complaints had alleviation or resolution of their symptoms after taking antimicrobial agents to which recovered plesiomonads were susceptible. These findings suggest that P. shigelloides may cause enteric disease in the normal host, that it may be acquired from eating uncooked shellfish, and that it may be a cause of travelers' diarrhea.