Mating barriers between genetically divergent strains of the parasitic nematode Haemonchus contortus suggest incipient speciation.

•We investigated the ability of genetically divergent Haemonchus contortus strains to interbreed.•Overlapping inter-strain morphological and morphometric variation was observed.•Co-infecting strains interbreed, but proportions of F1 progeny depart from Mendelian expectations.•Post-zygotic incompatibility in hybrid progeny of paternal parents is consistent with incipient speciation of MHco10(CAVR).•Our results have implications for ecology of anthelmintic resistant parasite populations.