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The formamidines are a structurally novel group of pesticides of growing importance in the control of mites, cattle ticks and certain orders of insects which have become resistant to conventional acaricides and insecticides. Their mode of action is complex with dose-dependent lethal and sublethal effects1–3. At sublethal levels they cause behavioural changes in the target pest species (for example in feeding and in mating behaviours), changes which are responsible for the protective effects on crops and livestock. Although many suggestions have been made for the underlying biochemical mechanism, including inhibition of monoamine oxidase activity, uncoupling of respiration and blockade of neuromuscular transmission1, no direct evidence has been presented. Another possibility is interaction with octopamine receptors in the central nervous system2,4,5. We report here that the formamidine acaricide/insecticide, chlordimeform (CDM), and its demethylated derivative can mimic the actions of octopamine at the locust neuromuscular junction. This gives the clearest evidence to date of the site of action of the formamidines and indicates a novel mode of action for these pesticides.