In order to complete growth and development, the endoparasitoid wasp,Cotesia (=Apanteles) kariyai, inhibits pupation of its armyworm host,Pseudaletia (=Leucania) separata.In host larvae retardation of testis and spermatocyst development caused by the parasitoid was also observed. The agents causing the retardation were found in the ovaries and venom of the female adult parasitoid. When an unparasitized male host larva was artificially injected with calyx fluid obtained from ovaries together with venom, it showed the same degree of developmental retardation of testes and spermatocysts as in natural parasitization. Testes implanted in isolated abdomens of healthy larvae did not increase in size by ecdysteroid stimulation after exposure to calyx fluid plus venom. It is suggested that both symbiotic polydnavirus existing in calyx fluid and venom in the parasitoid,C. kariyai, are responsible for the parasitic retardation of the male reproductive organs in the host,P. separata.