When mice, previously given oral inoculation with viable oncospheres of the heterologous cestode species (Hymenolepis diminuta, H. microstoma, Taenia taeniaeformis) and the homologous one (H. nana), were challenged with oncospheres of H. nana 4 days after the primary inoculation, they showed strong and complete resistance to H. nana challenge, respectively. However, the resistance was not evoked in mice given either infective eggs of Toxocara canis or non-viable oncospheres of all cestode species examined. Congenitally athymic nude mice given viable oncospheres did not show any resistance to H. nana either. Eosinophil infiltration around cysticercoids of H. nana in the intestinal villi appeared to be more prominent in mice previously given viable oncospheres of H. diminuta than in mice given non-viable oncospheres or PBS only. Some of the eosinophils in the villus harboring cysticercoid(s) of H. nana invaded the epithelia in the former, whereas all eosinophils remained in the lamina propria in the latter. There was almost no eosinophil infiltration in nude mice. Microscopic observations revealed that oncospheres of H. diminuta, which require beetles as the intermediate host like H. microstoma, could invade the mouse intestinal tissue. Therefore, it is strongly suggested that the strong cross resistance to H. nana in mice, induced by oncospheres of all heterologous cestode species, is thymus-dependent and due to oncospheral invasion into the intestinal tissue of mice.