On St. Lawrence Island, U.S.A. (Bering Sea), following definition of the cycle of Echinococcus multilocularis and its identification as the cause of alveolar echinococcosis, we intensified the investigation of helminths in the northern vole (Microtus oeconomus), the major intermediate host of that cestode. Natural infections of the voles by larvae of Toxascaris leonina were first observed by us on the island in 1956, and subsequently on the mainland of Alaska. The natural final host is the arctic fox; sledge dogs were kept in large numbers by the Yup'ik residents of the island, and voles were most commonly affected where dogs were present. Larvae of the nematode were often macroscopically visible in the walls of the cecum and in skeletal muscle. Results of experimental infection of laboratory-reared voles, with the use of embryonated eggs of nematodes obtained from dogs, indicated that the larvae tended to migrate anteriad in voles, from the cecum to muscles anterior to the diaphragm. Repeated exposure of voles apparently intensified the immune response to the extent that larvae from exposure subsequent to the first were retarded in development and might have been prevented from completing migration to the musculature. The possibility of vole-to-vole transmission of third-stage larvae (determined by feeding of somewhat dried eviscerated bodies of infected animals) was confirmed in 11 of 29 trials. Because the rather high prevalence of eosinophilia in the people was not correlated with cases of alveolar echinococcosis (formerly hyperendemic on the island), we consider that the larva of T. leonina was its probable cause.