The abundance of the intestinal helminth Trichostrongylus retortaeformis in adult mountain hares (Lepus timidus) from a moor in northeastern Scotland was assessed monthly. Weight and fatness of each hare was measured and the reproductive output of females was estimated by sectioning ovaries. Abundance of the parasite was lower in December and January than at other times of year, and there was no difference in abundance between male and female hares. There was no correlation between intensity of infection with T. retortaeformis and weight or fatness of hares either at the onset of the reproductive period (February and March) or at its termination (August and September). We found no relationship between fecundity of female hares and parasite abundance.