Yersinia enterocolitica comprises both pathogenic and nonpathogenic members. Distinguished by biogrouping, serogrouping, and ecological distribution, commonly occurring pathogenic serobiogroups, e.g., O:3/4; O:5,27/2; O:8/1b; O:9/2, possess both chromosomal and plasmid-mediated virulence traits. Studies have revealed several (oral, blood transfusion) modes of acquisition, elucidated the putative role of chromosomal and plasmid-encoded virulence factors in the pathogenesis of human infection, and have identified major animal reservoirs, e.g., the pig. Diagnosis has been refined though use of selective media, monoclonal antibodies directed against outer membrane proteins, and of purified yersiniae outer membrane proteins for antibody detection. Epidemiological investigations of foodborne outbreaks have been advanced through the use of molecular biology techniques such as ribotyping and pulsed-field gel electrophoresis.