Venustatriol, a new tetracyclic ether derived from squalene, which has anti-viral activity, has been isolated from the red alga Laurencia venusta. Its structure and absolute configuration were determined by X-ray using the absolute structure parameter x. By comparison of their spectral properties, the absolute configuration of its congener, thyrsiferol, was deduced. The absolute configuration of venustatriol was determined by X-ray.