Mice inhabiting the Nat. Hist. building, Urbana, Illinois, were found to harbor O. ratti, hitherto reported only from rats taken on ocean vessels in Japan. Studies revealed that both laboratory rats and mice may serve as definitive hosts, that they cannot be infected directly, and that various insects are capable of serving as intermediate hosts Trogoderma versicolor, Attagenus piceus, Anthrenus verbasci, Tribolium confusum, T. ferrugineum, Tenebroides mauritanicus and Plodia interpunctella. Insects tested but found unable to transmit the parasite are: Blatta orientalis, Tenebrio molitor and T. obscurus. Oribatid mites, probably Galumna, also failed to become infected. Onchospheres of O. ratti ingested by the larval insect are freed of their outer shell and penetrate the ventriculus within 48 hrs., whence they gain entrance into the haemocoele and rapidly develop into cys-ticercoids. The latter become infective to mice or rats by at least the 18th day within the insect and may remain infective throughout the insect''s metamorphosis for a period at least as long as 143 days. Development of the tapeworm within the mouse was studied at intervals from the time of ingestion of the cysticercoid until gravid proglottids were passed; a period of 23 days.