Fasciculin, a 6750-Da peptide from the venom of Dendroaspis, is known to inhibit reversibly mammalian and fish acetylcholinesterases at picomolar concentrations, but is a relatively weak inhibitor of avian, reptile, and insect acetylcholinesterases and mammalian butyryl-cholinesterases. An examination of fasciculin association with several mutant forms of recombinant DNA-derived acetylcholinesterase from mouse shows that it interacts with a cluster of residues near the rim of the gorge on the enzyme. The aromatic residues, Trp286, Tyr72, and Tyr124, have the most marked influence on fasciculin binding, whereas Asp74, a charged residue in the vicinity of the binding site that affects the binding of low molecular weight inhibitors, has little influence on fasciculin binding. The 3 aromatic residues are unique to the susceptible acetylcholinesterases and, along with Asp74, constitute part of the peripheral anionic site. Fasciculin falls in the family of three-loop toxins that include the receptor blocking alpha-toxins and cardiotoxins. From this basic structural motif, a binding site has evolved on fasciculin to be highly specific for the peripheral site on acetylcholinesterase. Acetylthiocholine affects rates of fasciculin binding at concentrations causing substrate inhibition. In the case of the mutant cholinesterases where rates of fasciculin dissociation are more rapid, steady state kinetic parameters also show acetylthiocholine-fasciculin competition to be consistent with occupation at a peripheral or substrate inhibition site rather than the active center.