none The extent to which the parasitoid Cotesia glomerata (L.) (Hymenoptera: Braconidae) discriminates between parasitized and unparasitized host larvae was tested by comparing the mean clutch sizes laid in thirty unparasitized and thirty parasitized two day-old Pieris brassicae (L.) (Lepidoptera: Pieridae) larvae. No eggs were laid in any of the previously-parasitized hosts. In contrast, eggs were laid in all but two of the unparasitized hosts. Most host-rejections involved only a brief insertion of the ovipositor, lasting two seconds or less. However, nearly a third occurred after a more prolonged insertion of the ovipositor, with a mean duration of twenty seconds: not significantly different from that of a normal oviposition.