The report which follows deals with some of the factors which promote or inhibit in vitro hatching of oncospheres of Hymenolepis nana and H. citelli. Because H. nana oncospheres are normally able to hatch in mammalian hosts as well as in insect hosts, experiments were devised to approximate these two environments by the use of beetle extracts or substances normally found in the mammalian intestine. Hymenolepis citelli, which requires an insect intermediate host, was studied to determine differences or similarities in egg structure and hatching behavior between this species and H. diminuta.