Synthetic inhibitors of acetylcholinesterase (AChE) recently have attracted particular attention for treatment of Alzheimer's disease. By systematic screening of microbial metabolites, we were able to discover the new AChE inhibitors, named arisugacins A and B, from the culture broth of Penicillium sp. FO-4259. The structures of arisugacins are members of the meroterpenoid compounds. Arisugacin A is a potent and highly selective inhibitor of AChE but does not inhibit butyrylcholinesterase in vitro. Arisugacin A is a good candidate as an excellent potential drug for treatment of Alzheimer's disease. Also reviewed is the current status of development of antidementia drugs.