We generated interleukin-5 receptor α chain (IL-5Rα)–deficient (IL-5Rα^−/−) mice by gene targeting. The IL-5Rα^−/− mice showed decreased numbers of B-1 cells concomitant with low serum concentrations of IgM and IgG3. They showed no IL-5-induced enhancement of B cell responses to T-independent antigens. The number of αβ T cell receptor–positive thymocytes tended to decrease in 3-week-old IL-5Rα^−/− mice, returning to normal by 6 weeks of age. The IL-5Rα^−/− mice produced basal levels of eosinophils, while their bone marrow cells failed to form eosinophilic colonies in response to IL-5. Impaired eosinophilopoiesis in IL-5Rα ^−/− mice enhanced the survival of Angiostrongylus cantonensis. These results indicate that IL-5-induced eosinophils serve as potent effector cells in the killing of Angiostrongylus cantonensis in mice.