In 1987 a cyanobacterium (blue-green alga) Umezakia natans was isolated from Lake Mikata, Fukui, Japan, as a new member of the family of Stigonemataceae. The crude extract of U. natans showed hepatotoxicity to mice, from which a toxic compound was isolated. The toxin was identical in all respects to a recently reported hepatotoxin, cylindrospermopsin, isolated from an Australian tropical cyanobacterium Cylindrospermopsis raciborskii. Because cylindrospermopsin causes fatty liver and central necroses in mice and is suspected of being an agent causing human hepatoenteritis, its monitoring in drinking water supplies has been required. So a rapid screening method including four steps, extraction, clean-up, separation, and determination, has been proposed for cylindrospermopsin. A combination of a clean-up using HP-20 and C18-cartridge, and HPLC with photodiode array detector made it possible to establish a screening method for the toxin. The established method was applied to five samples and cylindrospermopsin was traced in one of them.