There is urgent need for additional insecticides to control the spread of malaria and other diseases. In Anopheles gambiae, two genes (ace-1 and ace-2) code for acetylcholinesterase (AChE); only one of these is related to target site resistance (ace-1), confirming its toxicological relevance for anticholinesterase activity. Molecular analysis of the ace-1 gene in light of the crystal structure of Torpedo californica AChE, finds an active site gorge leading to the catalytic site, adjoining an oxyanion hole. This enzyme also contains the so-called peripheral site that is near the external mouth of the gorge. In rat, both the peripheral and catalytic site interact simultaneously with bivalent tacrines, potent inhibitors of rat AChE. We have undertaken a systematic screening of bivalent tacrines having methylene linkers from 2-12 carbons in length. In human, the optimum tether length is 7 carbons (IC50=1.6 nM), while in An. gambiae, the optimum length is 4 carbons (A4A; IC50=163 nM). Neither the hydrochloride salt nor free-base of A4A was toxic to mosquitoes via surface contact exposure. A4A (1 µM) applied to the transected central nervous system (CNS) of larval Drosophila melanogaster caused rapid firing in peripheral nerves, while the same treatment applied to intact CNS caused no firing, indicating that poor penetration was a probable cause of low toxicity, in vivo. Overall, these compounds are less potent and display less tether length dependency at Anopheles AChE, compared to human or rat. The implications of this structure-activity relationship for insecticide design will be discussed.