Ion channels are targets for many drugs including insecticides and anthelminthic agents such as ivermectin (IVM) and moxidectin (MOX). IVM has been shown to activate glutamate-gated chloride channels (GluCls) from the free-living nematode, Caenorhabditis elegans. Haemonchus contortus is a parasitic nematode that is also extremely sensitive to IVM. The high sensitivity of H. contortus to IVM is probably the result of the fact that, like C. elegans, H. contortus also express GluCls. To investigate the potential physiological response to IVM in H. contortus we have expressed a GluCl from this parasite (H. contortus glutamate-gated chloride channel, HcGluCla) in Xenopus oocytes. HcGluCla expressed in oocytes formed a homomeric channel that responded to glutamate and ibotenate as well as the anthelmintics IVM and MOX. The response to glutamate and ibotenate was fast acting and reversible whereas the response to IVM and MOX was a slower activating channel that was essentially irreversible. These results suggest that IVM toxicity in H. contortus is the result of its irreversible activation of GluCls.