Structural aspects of insect pheromones, especially their hydrophobicity, prompted the examination of them byin vitroandin vivoexperiments as inhibitors of acetylcholinesterase. All 17 pheromones assayed in sufficient detail were reversible, competitive inhibitors of acetylcholinesterase:Kivalues ranged from 0.27 mM for frontalin to 9.3 mM for ipsenol, both aggregation pheromones. All seven pheromones assayed withTribolium castaneumimmobilised this species with an effect that was weakly dose-responsive over the range of concentrations tested. Concentrations eliciting 25% immobility (IC25) ranged from 2.0x103ppm for 3-methy 1–2-cyclohexenone to 2.0x104ppm forcis-7, cis-11-hexadecadien-1-yl acetate, both sex attractants. Such efficacies suggest that the pheromones tested would be effective as defence agents only in species living in a confined area, as under bark, or in aggregations.