Examination of the throat flora from swine in Ontario for Yersinia enterocolitica found the incidence of serotype O:3 to vary from 20% from tonsils to 50% for throat swabs and 55% for tongues. In contrast, there were no isolations of serotype O:3 from throat swabs taken from swine in the western provinces of British Columbia and Alberta. This incidence of serotype O:3 in swine correlates well with the human incidence of the same serotype which is 81% for all human isolations of Y. enterocolitica in the eastern provinces and 4% in the western provinces of Canada. The opposite relationship is true for serotype O:5,27, which was not isolated from Ontario swine but occurred with a frequency of 3% in British Columbia swine and 24% in Alberta swine. The relative incidence in humans for serotype O:5,27 is 5% of all isolations of Y. enterocolitica in Ontario, 8% in British Columba, and 26% in Alberta. Serotype O:8, the most common human serotype in the western provinces (40% of all isolations), was not isolated from swine in this survey. The majority of Y. enterocolitica cultures of biotype 4 (serotype O:3) and biotype 2 (serotype O:5,27) were positive for autoagglutination, a test which has been associated with virulence, whereas all cultures of biotype 1 were negative. The results from this study strongly suggest that swine are an important source of human infections with both serotype O:3 and O:5,27 of Y. enterocolitica. The source of human infections in western Canada with serotype O:8 remains unidentified.