The abomasa of 40 wild ruminants (18 red deer, 9 roe deer, 13 European bison) shot in the Bieszczady Mountains and another 8 European bison shot in Bialowieza Primeval Forest, were examined for worms. Ashworthius sidemi was found in all of the animals from the Bieszczady Mountains, and the intensity of infection was usually massive. In Bialowieza Forest few specimens of A. sidemi were noted among three bison. Wild ruminants were infected by A. sidemi from June to September. The nematodes originating from this infection only attained sexual maturity in June of the following year, and from September the process of expelling the parasites of that generation began. From June to autumn, two generations of nematodes, one consisting of mature individuals from the previous year and the other of the current year comprising fourth stage larvae as well as juveniles, occur in these hosts. In the winter and spring periods until June, only one generation of the parasite, arrested fourth-stage larvae and juveniles, occurs in the hosts.