A reference strain of Mycobacterium avium subspecies paratuberculosis was added to faecal larval cultures of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis. Samples of the larvae produced were cultured for the presence of the bacterium in modified BACTEC 12B medium, both before and after exposure to gamma irradiation. The water used to wash the larvae off the faecal cultures was also tested for the presence of the bacterium. Positive growth was confirmed as M avium subspecies paratuberculosis by IS900 polymerase chain reaction and restriction endonuclease analysis of the product. M avium subspecies paratuberculosis was detected in the unirradiated larval suspensions and wash waters of all three nematode species, and in the irradiated H contortus larval suspension.