Taenia crassiceps was common in Vulpes fulva examined from southern Ontario. Metacestodes occurred naturally in Microtus pennsylvanicus, Marmota monax, Tamias striatus, and Ondatra zibethicus, and Peromyscus maniculatus, Tamiasciurus hudsonicus, and Sciurus carolinensis were infected experimentally; all rodents are new host records. Cysticerci developed into adults in dogs or foxes within 5 to 6 weeks; five coyote pups resisted infection. Development of the metacestode was followed mainly in white mice. Infections were most common subcutaneously, but also occurred in both body cavities. Mice approximately 4 weeks of age were most susceptible. Asexual reproduction occurred by exogenous, and rarely endogenous, budding from the abscolex pole beginning approximately 3 weeks after infection. Metacestodes in various stages of development were injected into mice subcutaneously, intrapleurally, but mainly intraperitoneally. Subsequent development and reproduction were similar to that following infection with eggs. Apparently all metacestodes are capable of budding. The initial rate of reproduction was higher subcutaneously and intrapleurally than intraperitoneally, but within approximately 100 days the rate became higher and continued higher intraperitoneally than elsewhere. Reproduction never reached a logarithmic rate. Metacestodes inoculated serially up to 21 times at 50-day intervals increased greatly in size and continued budding. Four other series were maintained by serial subinoculation at 50-day intervals through 23 generations without a significant change in the rate of reproduction.
