THE resistance of ticks to chemical methods of control is an increasingly serious problem1–5, and new classes of compounds with high acaricidal activity and low mammalian toxicity are needed urgently. In the search for novel acaricides the nervous system is a prime target, and, in spite of evidence about the role of γ-aminobutyrate and gluta-mate in arthropod nervous transmission, acetylcholine is the only well established neurotransmitter6–9. So far only acetylcholinesterases have achieved an important role in the disruption of cholinergic neurotransmission. Anti-cholinergics, such as atropine, are ineffective, and cholino-mimetics other than nicotine have been little investigated. We now report that certain muscarinic agonists are potent acaricides.