Acetylcholinesterase is an enzyme responsible for the timely termination of acetylcholine action at muscarinic and nicotinic receptors, so it is essential for the normal function of cholinergic synapses. Naturally occurring acetylcholinesterase inhibitors are relatively rare, but several synthetic acetylcholinesterase inhibitors have been developed as potent remedies, pesticides, and neurotoxins. Here we describe the inhibition of soluble and membrane-bound cholinesterases with a recently isolated and purified acetylcholinesterase inhibitor isolated from the crust coral Parazoanthus axinellae Adriaticus. This substance with a novel chemical structure might serve as a new prototype of cholinesterase inhibitors.