Bacteriologic data and some clinical and epidemiologic aspects concerning 53 clinical cases associated with Yersinia enterocolitica are described. This is the first published report of human cases of Y. enterocolitica infection in Israel. Seventy-eight isolates of Y. enterocolitica were obtained from stool cultures of these cases, with 94% on SS agar plates. The isolation of the organism, including its differentiation from related bacteria, is discussed. In 88% of 50 cases, the organisms were of serotype 0:3, biotype 4, phage type VIII and in 12%, serotype 0:1, 2a, 3, biotype 3, phage type XI. Sensitivity in vitro to antibacterial drugs and evaluation of the serologic diagnosis are also considered. With two exceptions, all 53 patients had acute enteritis, clinically similar to shigellosis. The cases came from towns and villages throughout northern Israel and were detected from July 1976 to November 1977. Children under five years of age were 76% of the cases. Five cases appeared in two small institutional outbreaks. It is suggested that infection probably occurs in other parts of the country as well, and that difficulty in laboratory diagnosis is the reason for the lack of reported cases.