Thirty-three strains of Yersinia enterocolitica representing 11 different serotypes were identified from human isolates in California between 1968 and mid-1977. The most frequently encountered serotype, 0:5,27, was identified in 8 cases. Twenty-one strains, representing five serotypes, were esculin- and salicin-negative, and were considered to be the primary cause of disease in the majority of these cases. The remaining strains, representing six serotypes, were esculin- and salicin-positive, and appeared to be 'opportunistic' organisms. They were isolated from 12 patients, 8 of whom had underlying conditions, and 4 or whom were asymptomatic. From 1968 to 1977, 13 strains of Y. pseudotuberculosis isolated from humans were submitted to our laboratory for identification or confirmation. Ten were serotype I and three, serotype III. Sources of isolation included blood (6); blood and liver abscess (1); blood and urine, and liver at autopsy (1); feces (2); biopsy of terminal ileum (1); abdomonal abscess (1); and neck abscess (1). The results of serotyping and biochemical characteristics of the strains, as well as the clinical histories of the patients, are discussed.