The helminth fauna of the wood mouse, Apodemus sylvaticus, in the Erro River valley (Navarre, Spain) was investigated from a total of 150 mice between February 2001 and July 2002. An overall prevalence of 90.7% was recorded and up to 14 helminth species identified. The most prevalent species was the nematode Heligmosomoides polygyrus (78.0%), whereas Syphacia stroma was the species with the highest median abundance (19.8). The detection of Calodium hepaticum, Rodentolepis straminea and the larvae of Hydatigera taeniaeformis are significant, since these helminth species could be considered potential human parasites. The helminth infracommunity comprised no more than five species. A significant predominance of monoxenous species was detected. Statistically significant differences were also found between prevalences, helminth abundance, species richness and helminth diversity of sub-populations of the wood mouse determined by host age and season of capture, which agree with most of the studies carried out on this host. This study will shed light on the helminth community of the wood mouse from a region of Spain which has not previously been documented.