Compared to mainland Britain, where there has been decades of anthelmintic use, the natural host-parasite relationship of the wild Soay sheep on the remote archipelago of St. Kilda has remained undisturbed. Small-scale anthelmintic bolus experiments on the island have previously shown that the removal of gastrointestinal nematodes can improve overwinter survival of young and male sheep in high host density years. This study, in which two-year-old sheep were treated, is the first to examine patterns of re-establishment of different nematode species in sheep following treatment and also investigates which species are likely to affect host survival. The experiment showed that, although all sheep were equally likely to die, host sex and weight influenced temporal survivorship in that females and heavier sheep tended to survive longer. Examination of the nematodes that had re-infected males six months after administration of an anthelmintic bolus showed that, compared to controls, the diversity of species was lower. Of the nine nematode species, there were fewer Trichostrongylus axei and Trichostrongylus vitrinus nematodes in treated sheep. In control sheep there were more T axei and T. vitrinus nematodes in males than females. In addition there was an association between host over winter weight loss and the intensity of T. vitrinus. The fact that this species had higher numbers in males than females and was associated with over-winter weight loss implies it could have an important role in host mortality in high-density years. (c) 2008 Elsevier B.V. All rights reserved.