It is shown in this study that two species of nematodes: Teladorsagia circumcincta (Stadelmann, 1894) and T. trifurcata (Ransom, 1907), (Nemátoda, Trichostrongylidae, subfamily Ostertagiinae), have a similar range of host species. Furthermore, the abscence of reproductive barriers, as well as the fact that both forms soon reach stable proportions in the host population, indicate that T. trifurcata is a morph of T. circumcincta.