Yersinia enterocolitica--recognized as a distinct taxonomic entity in 1964--belongs to the family of enterobacteriaceae. It has been isolated with increasing frequency from human and animal sources as well as from food and non-chlorinated water. Yersinia enterocolitica can produce enteritis in man, accompanied or followed in adults by erythema nodosum, arthralgia and/or acute arthritis. Rarely, septicaemia with a high mortality rate has been encountered. A cold-temperature enrichment method was used to examine 1135 faecal specimens; 11 were positive for Yersinia enterocolitica. Symptoms of enteritis were reported by all 8 patients whose faeces contained the bacterium; a brief description is given of the course of illness in each patient. Biochemical and serological properties of the isolates are discussed with special reference to some unusual results obtained with the commerical API-20 E system for identification of enterobacteriaceae when incubated overnight at 35 degrees C.