In the course of our screening for microbial metabolites that inhibit the activity of acetylcholinesterase (AChE), we isolated a novel and selective inhibitor of AChE, arisugacin, from the culture broth of Penicillium sp. FO-4259. Arisugacin was co-produced with known tremorgenic compounds, territrems B and C, which were reported as the metabolites of Aspergillus terreus. This communication deals with production, isolation, physico-chemical properties and biological activity of arisugacin.