Parasitic wasps (Nemeritis canescens ) discriminated between hosts (larvae of the flour moth Plodia interpunctella ) containing their own progeny and those containing the progeny of other individuals when deciding whether or not to avoid superparasitism. This differential discrimination lasted for about 48 hours. The ability of a parasitoid to detect prior parasitism, and avoid subsequent superparasitism, seemed to depend on the operation of a specific chemical marker, originating in Dufour's gland, and introduced into the host with each egg laid. A specific mathematical model, based on game theory, supports the experimental data in indicating that the patterns of differential discrimination observed are evolutionarily stable. none