Development of the cysticercoid of Hymenolepis microstoma in the flour beetle Tribolium confusum is described from the hatched oncosphere to the fully formed cysticercoid infective to mice. At 30 C growth and differentiation are completed after 7.5 days. The sequence of stages in development includes the formation of a spherical cell mass with the production of an eccentric cavity, elongation of the sphere leading to a bipartite body, withdrawal of the narrow, anterior part of the body into the cavity with subsequent differentiation of scolex musculature and hooks, and growth of the cysticercoid tail. Similar growth and completion of development also occur in adults of the beetle Tenebrio molitor. Recent studies on the biology of Hymenolepis microstoma by Dvorak et al. (1961) showed that this cestode should be an interesting species for laboratory experimentation. The adult cestode lives in the bile ducts of mice and the cysticercoid develops in the confused flour beetle, as well as in other insect hosts. The histologic structure of the cysticercoid was described by Voge (1963). Because studies on the development in the intermediate host from the oncosphere to the infective cysticercoid have not been published, observations on this phase of the life cycle are presented here. MATERIALS AND METHODS The strain of Hymenolepis microstoma obtained from Rice University, Houston, Texas, was carried in Swiss-Webster albino mice. Laboratory-reared confused flour beetles were fed eggs from gravid proglottids passed in the feces of infected mice. For studies on development, beetles which had been starved for 1 week were allowed to feed on eggs for 4 hr at 29 to 30 C; they were then transferred to ground oats and maintained at 30 C, or at lower temperatures. Eggs were also fed to adult Tenebrio molitor. Beetles were dissected during the feeding period, and several times each Received for publication 6 June 1963. * Aided by a research grant (AI-01583-07) from the National Institutes of Health, United States Public Health Service, Bethesda, Maryland. day until completion of cysticercoid growth. All photographs and measurements were made from live material. Whole mounts of developmental stages were from Bouin's-fixed material stained with Harris' hematoxylin.