Sexual transmission occurs commonly in microparasites such as viruses and bacteria, but this is an unusual transmission route for macroparasites. Here we present evidence which suggests that a nematode parasite of Wood Mice (Apodemus sylvaticus) may be sexually transmitted and we have classified the nematode using molecular data. Wood Mice were collected annually in the course of work on their reproductive physiology. Larval nematodes were found in the epididymides of 19.6% of males. It seems likely that they would be transmitted to females at ejaculation. To identify these larval nematodes, which we were unable to do using morphological features, we sequenced the 18S rDNA. Sequence comparisons with the molecular phylogeny of Blaxter et al. (1998) demonstrated that they were bursate nematodes (Order Strongylida). The relationships between strongylid taxa were poorly resolved by 18S rDNA. However, both distance and parsimony analyses grouped the nematode with the superfamily Metastrongylidea in a clade containing Filaroides and Angiostrongylus sp. Importantly, the sequences were distinct from those of Heligmosomoides polygyrus and Angiostrongylus dujardini, two common strongylid nematodes of Apodemus. We were therefore unable positively to identify these worms by matching their sequences with those from morphologically identifiable adult strongylid nematodes infecting Apodemus. These results demonstrate that an as yet unidentified strongylid is quite commonly found in large numbers in the male reproductive tract of Wood Mice. Further work is required to understand the biology and transmission dynamics of this interesting system.