Four free-living European bison of the Bialowieza/Caucasus line which were shot in February 1997 within Lutowisko Forestry District in the Bieszczady range of the Eastern Carpathians, Poland, were all infected by Ashworthius sidemi at a mean intens ity of infection of 1542 worms. On the basis of study of nematodes in Cervus elaphus from France, typical examples of A. gagarini, and 629 specimens of Ashworthius collected from the Bieszczady bison, A. gagarini was recognized as a junior synonym of A. si demi.