Yersinia enterocolitica biotype I were isolated from faeces of 16% of 56 consecutive patients with diarrhoea or gastrointestinal symptoms and 2.8% of 109 healthy controls (p less than 0.01). Similar Yersinia biotypes were isolated from 4% of samples from 86 rheumatoid arthritis patients and 4.5% of samples from 140 ankylosing spondylitis patients examined regularly over an 8-month period. These results suggest that Yersinia enterocolitica biotype I are regular but infrequent inhabitants of the human gastrointestinal tract in south-east England. The increased isolation rate of these microorganisms from patients with enteric disease and from patients with exacerbations of HLA B27-related arthritic and ocular inflammatory disease justifies further investigations.