An account is given of the life-history of Pseudendaphis maculans Barnes, a common and widely distributed endoparsite of Aphids, especially of Toxoptera aurantii (Boy.), in Trinidad. Alates are parasitised to a much smaller extent than apterae : it is thought that this is due to the parasite larva inhibiting wing development. Nymphs appear to be attacked usually during the first stadium; the parasite larva is fully fed after 6 to 7 days. Shortly before the larve leaves its host the latter assumes a characteristic spotty appearance, due to numerous granules of dark matter that are readily visible through the integument, which has become paler and more transparent than normal. Pupation takes place in the soil: 9–10 days after leaving the host the adult emerges, invaribaly during the early afternoon.