During an 11-year period (1978 to 1989), over 300 strains of Yersinia spp. (excluding Y. pestis and Y. pseudotuberculosis) were recovered from a variety of gastrointestinal and extraintestinal sites in patients in California. Over the 11-year period, Y. enterocolitica serogroup O:3 predominated, although a shift in the relative frequency of this serogroup was observed during this interval, increasing dramatically during the years 1984 to 1989. Of the remaining Y. enterocolitica isolates, over 40% were identified as belonging to serogroups generally considered to be nonpathogenic, although many of these isolates were recovered in association with milder cases of gastroenteritis. The results suggest a changing and expanding spectrum of Y. enterocolitica serogroups associated with various gastrointestinal and systemic infections.