The role of semiochemicals in mediating intraguild interactions between the seven-spot ladybird, Coccinella septempunctata, and the aphid parasitoid, Aphidius ervi, was investigated. Female parasitoids avoided leaves visited by C. septempunctata adults and larvae during the previous 24 hr. Ethanol extracts of C. septempunctata adults and larvae also induced avoidance responses by A. ervi. Two of the hydrocarbons identified by gas chromatography ( GC) and coupled GC-mass spectrometry (GC-MS), n-tricosane (C23H48), and n-pentacosane (C25H52), when tested individually at levels found in the adult extract, induced avoidance by A. ervi. Further investigation of the larvae extract, and footprint chemicals deposited by adults in glass Petri dishes, confirmed the presence of the hydrocarbons. Parasitism rates of the pea aphid, Acyrthosiphon pisum, on broad bean plants, Vicia faba, which had been sprayed with a mixture of the chemicals, were lower than those on control plants. The effect, however, was no longer evident if parasitoid foraging was delayed by 24 hr after the plants were treated. The ecological significance of intraguild avoidance behavior and implications for possible use of the semiochemicals involved in future biological control programs are discussed. Nutrition; Diet; Prey; Behaviour; Communication; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Predators; Competition Coccinella septempunctata (Coccinellidae); Insect prey; Acyrthosiphon pisum (Hemiptera); Chemical signals; Semiochemicals; Role in interaction with intraguild hymenopteran parasite of hemipteran prey; Interspecific competition Aphidius ervi (Aphidiidae); Chemical signals; Semiochemicals; Hemipteran hosts; Acyrthosiphon pisum; Semiochemicals role in avoidance of intraguild coleopteran predator during host searching; Interspecific competition Acyrthosiphon pisum (Aphidoidea); Hymenopteran parasites; Aphidius ervi; Intraguild interaction of parasite & coleopteran predator, role of semiochemicals; Insect predators; Coccinella septempunctata (Coleoptera) none