The helminths of the alimentary tract of wood mice, Apodemus sylvaticus, were studied at two sites over a 33-month period. Nine helminth species were recovered regularly. All but one was absent for at least 1 month. Monthly samples from these helminth communities were more similar to samples taken in the following month than to samples taken at progressively greater intervals up to a year. Helminth communities at both sites, however, had cyclical elements. Comparison of mean similarity indices for helminth faunas from 33 monthly samples and those from data sets generated by four null models suggests that observed values did not differ from a model where relative abundance of each species was determined randomly with specific maxima of abundance and species absences based on observed data. Loss of helminth species may decrease measurements of community stability based on relative abundance while persistence of abundant species increases stability. Helminth faunas in samples of A. sylvaticus from six localities taken at the same time of year at 5-year intervals indicated that some changed radically while others remained virtually unchanged. The considerable variation in helminth communities from different localities was not related to proximity or gross habitat characteristics. Nematospiroides dubius, Corrigia vitta and Capillaria murissylvatici were important in discriminating between the parasite faunas at different sites. Abundance of Syphacia stroma varied considerably between spatial surveys reflecting differences in host population dynamics in the 2 years. The present report and data from elsewhere in Ireland suggest that species composition of the helminths associated with A. sylvaticus may be stable over a wider geographical scale. There were neither strong nor consistent positive or negative interactions between pairs of helminth species. It is concluded that the stability characteristics of this parasite community, in terms of species composition and relative abundance, are the product of the population biology of independent parasite species rather than interspecific interactions. Variation in the role of competition in parasite communities is discussed.