Ovipositing females of the parasitoid Microplitis demolitor discriminated between larvae of two major Australian lepidopteran pest species, Helicoverpa armigera (corn earworm) and Helicoverpa punctigera (native budworm). Parasitoid populations from different geographical locations differed in their host preferences. The pattern of variation showed consistent change of preference with latitude and suggested that evolution of this preference may be driven by the hosts' local abundance and seasonal phenology. Microplitis demolitor showed a strong preference for second- and third-instar host larvae over all other host larval instars. This preference probably represented a compromise between the improved host survivorship provided by older larvae and the low risk of injury to the ovipositing parasitoid when attacking younger larvae. none