Setting aside the host specificity, the food habit is the most important of the various ecological factors concerning voles. The herbivorous vole has less chance of being infected with cestodes than the insectivorous vole. No sex resistance of voles to helminth infections was revealed. The increase of incidence of helminths with the progress of host's age was observed in the cases of helminth species, Paranoplocephala omphalodes, Coronacanthus apodemi, Heligmosomum yamagutii, Longistriata hokkaidensis, Mammaniduloides hokkaidensis and Capillaria hepatica. This phenomenon can be explained by the increased chances of infection with the progress of the host's age. The seasonal analysis of helminth fauna was discussed, classifying the year into 5 periods. Younger animals play a main role in the abrupt reduction of the incidence of H. yamagutii. Older voles cause a higher increase in the incidence of C. hepatica and M. hokkaidensis. A high incidence of C. apodemi and Plagiorchis muris in the summer is attributable to the abundance of invertebrate intermediate hosts in this season. The increase of Syphacia montana in the autumn and spring originates in the increase of opportunities for contact of voles in the reproductive seasons. In two species, C. apodemi and C. hepatica, significant differences between the two experimental areas were proved about their incidences. The difference of arthropod fauna by area is suspected concerning these helminths, since they require arthropods as intermediate hosts or disseminators in their life cycles.