none Testis development in P. brassicae (L.) parasitized as 1-day-old larvae by A. glomeratus (L.) is retarded considerably from the 2nd-instar onward. Testicular volume is only 1/6 and testicular wall thickness only 1/4 that of the normal testes at the end of the 5th larval instar. Nevertheless, bundles of sperm are formed in parasitized larvae. After interrupted oviposition, leading to the lowest possible level of parasitism, the effects are of the same magnitude as in more heavily parasitized hosts at comparable age. No significant correlation betwen number of parasitoids and retardation inflicted is found. The possible source of the effect of parasitism is discussed.