Aragusterol A, a newmarine steroid possessing potent antitumor activity, was isolated from the Okinawansponge of the genusXestospongia and its structure was determined by spectroscopic analysis and chemical evidence. The compound strongly inhibited the cell proliferation of KB, HeLaS3, P388 and LoVo cells in vitro, and also showed potent in vivo antitumor activity toward P388 in mice and L1210 in