Reciprocal crosses between an isolate of Haemonchus contortus resistant to both benzimidazole and levamisole anthelmintics and a susceptible isolate were performed in order to determine the mode of inheritance of these resistances. F1 and F2 generations and parent isolates were assayed for susceptibility to thiabendazole and levamisole in vitro. For each drug all of the filial generations were intermediate in susceptibility between the parent isolates, and analysis indicated that resistance was inherited as an incompletely recessive character determined by more than one gene in each case. There was no evidence of maternal inheritance. Results of both the in vitro assays and in vitro selection, followed by determination of sex ratio in the survivors, as well as studies on adult worms, provided no evidence for sex-linkage. This work illustrates that in vitro assays coupled with minimal studies in sheep are useful for determining inheritance of resistance, yet use fewer experimental animals than traditional studies.