A total of 648 voles (Microtus p. pennsylvanicus M. ochrogaster and M. p. drummondii) collected over a period of 6 years from the North Central States and Manitoba have been examined for helminth parasites. Special reference has been made to host-parasite relationships, and detailed information has been given for 2 areas where intensive studies were made. A total of 21 spp. of helminths was collected (Andrya macrocephala, A. microti, Andrya sp., Paranoplocephala troeschi, Paranoplocephala spp., Choanotaenia sp., Hymenolepis fraterna, H. evaginata, Hymenolepis sp., Taenia taeniaeformis, Cladotaenia sp., Quinqueserialis hassalli, Mediogonimus ovilacus, Entosiphonus thompsoni, Nematospiroides sp., Longistriata dalrymplei, Dictyocaulus viviparus, Syphacia obvelata, Mastophorus muris, Capillaria muris-sylvatici and Trichuris opaca). The helminths encountered have been discussed separately. Information has been given on seasonal variation in infections with certain helminths. There is evidence that season of the year and geographical location have much influence on kind and number of helminths parasitic in voles. As far as could be determined, most of the vole parasites have little or no harmful effect upon the host.