Females of Muscidifurax zaraptor Kogan & Legner, a solitary pupal parasitoid of house flies, Musca domestica L., discriminated between hosts previously parasitized by themselves and by conspecifics. When hosts were in short supply, females avoided self-superparasitism and laid eggs in hosts that had been previously parasitized by conspecifics. When females were given a choice to oviposit in unparasitized pupae and hosts previously parasitized by themselves and conspecifics, females avoided superparasitism. Superparasitism seems to be avoided by the use of short-term memory of the position of the pupae and the use of internal host cues. Females did not respond to external cues. The adaptive significance of superparasitism in M. zaraptor is discussed.