The recent discovery of a larval nematode in the epididymides of free-living wood mice (Apodemus sylvaticus) suggests a sexual transmission of these parasites. They have been placed within the bursate nematodes (order Strongylida) through 18S rDNA analysis, suggesting that they are undetermined metastrongyloid nematodes. The possibility that these parasites are transmitted sexually opens an intriguing field of research because sexually transmitted metazoan parasites are known to occur mainly in invertebrates, whereas in vertebrates sexually transmitted parasites are usually microparasites such as viruses, bacteria and protozoa.