Seasonal fluctuations in the prevalence and intensity of infection with Heligmosomoides polygyrus (Nematoda) were studied in Apodemus sylvaticus (wood mouse, n = 399), sampled from three contrasting habitats in southern England, to test the hypothesis that both intrinsic (host sex, age) and extrinsic (season, site) factors influence parasite burdens. Maximum likelihood techniques based on log linear analysis of contingency tables were employed to generate a minimum sufficient model for prevalence of infection and 3-way ANOVA with negative binomial errors was used to evaluate the relative contribution of the principal factors and their interactions in explaining variation in worm burdens. Host age could not be entered into either statistical model because of some incomplete subsets of data. However, it was evident that in general juvenile mice carried lower worm burdens compared with adults, although these increased towards the winter season. Host sex was not a significant factor, other than in making a contribution to a weak significant interaction between sex and site, arising from male bias in one site and female bias in the remaining two. The principal determinants of variation in worm burden were the extrinsic factors, site and season, with an approximately equal weighting, and their interaction. These effects arose because worm burdens were lower at Dungeness and showed quite different seasonal patterns to the Egham and Isle of Wight sites. We propose that the unique character of the Dungeness habitat was not conducive to optimal transmission of H. polygyrus throughout most of the year (excepting spring) and we suggest possible explanations for these observations.