Aphidius ervi Haliday is an important parasitoid of several aphid species, and information is needed for the development of mass-rearing techniques and better understanding of biological control ecology. The emergence, sexual maturation and oviposition of A. ervi on pea aphid, Acyrthosiphon pisum (Harris), was studied in the laboratory at 20[plus or minus] 1[degree]C and 60-70% RH with 16:8 h light: dark. About 95% of parasitoids emerged during the photophase. Females needed a significantly longer time than males to complete their life cycle. Newly emerged males were able to perform their courtship display but failed to mate until they were 4 h old; newly emerged females were able to respond to males? courtship display and mate. Females attacked aphids in both light and dark conditions. The number of eggs laid and parasitism (number of aphids parasitised) per oviposition bout (2 h oviposition period) were significantly greater in the photophase than in the scotophase. none