A series of experiments was conducted to determine if hybridization could occur between members of 2 species of nematodes, Cooperia oncophora and C. pectinata, parasitic in ruminants. Fourth-stage C. oncophora of one gender and C. pectinata of the opposite gender in the same stage of development were injected into the duodenal lumina of helminth-free calves. Infective third-stage larvae grown from eggs passed in the feces of the surgically infected calves were used to infect orally other helminth-free calves. Adult F1 generation hybrid parasites had morphological characters of both pure- bred species. One spicular character was unique to hybrid nematodes. Attempts to establish adult F2 generation hybrid parasites were unsuccessful. In experiments attempting to determine whether hybridi- zation could occur under simulated natural conditions, calves were administered infective third-stage larvae of both species and genders in sequences best designed to favor interspecific matings. Hybrid nematodes were not observed when adult progeny of these matings were examined. The experiments described apparently constitute the first successful hybridization of nematodes under controlled experi- mental conditions.