Abstract
Females of Spalangia cameroni Perk, use the ovipositor to differentiate between unparasitized and parasitized housefly (Musca domestica L.) pupae; and prefer to lay on unparasitized hosts instead of those attacked by S. cameroni, Nasonia vitripennis (Walk.), or Muscidifurax zaraptor K. & L. (Hymenoptera: Pteromalidae). The intensities of oviposition restraint on the three kinds of attacked hosts are similar. Changes in parasitized pupae to which the female parasites react probably involve "venoms" injected by each species during oviposition.