The effects of abundance, age, and sex of feral domestic mice Mus musculus domesticus on infections with the nematode parasite Syphacia obvelata were analyzed during a long-term study of the mouse population on Guillou Island (1.45 km²), a part of the subantarctic Kerguelen Archipelago. The population dynamics of the nematode did not follow the variation in host abundance. However, depending on the year, differences in pinworm abundance were found between the age classes and sex. Such patterns suggest that parasitic infections may have been modulated by host-intrinsic factors, e.g., either by the way of innate or adaptive immunity, rather than extrinsic factors, e.g., host abundance.