A sensitive assay based on amplification of a 319-bp DNA fragment of the intracellular bacterium of swine proliferative enteritis was developed for the detection of the organism in the feces of swine. A vernacular name, ileal symbiont intracellularis (IS-intracellularis), has recently been published for the intracellular bacterium, which was formerly known as a Campylobacter- like organism (C. J. Gebhart, S. M. Barnes, S. McOrist, G. F. Lin, and G. H. K. Larson, Int. J. Syst. Bacteriol. 43:533-538, 1993). As few as 101 IS- intracellularis organisms purified from intestinal mucosa, or 103 IS- intracellularis per g of feces, were detected. No amplification product was produced from a polymerase chain reaction performed on DNA extracted from the feces of healthy pigs. A 319-bp DNA fragment specific for IS-intracellularis was produced on amplification of DNA from the feces of pigs with experimental and naturally occurring proliferative enteritis.