Pilosanones A (1) and B (2), the major diterpenoid constituents of Portulaca pilosa L., have been assigned structures with a novel bicyclo[5.4.0]undecane skeleton on the basis of detailed spectroscopic analyses, and their significance in the biosynthesis and the chemosystematics of Portulaca plants is suggested. We report here the isolation and structure elucidation of pilosanones A (1) and B (2), the major constituents of Portulaca pilosa L., which have a novel bicyclo[5.4.0]undecane skeleton and could be significant in relation to the biogenesis and chemosystematics of the diterpenoids in Portulaca plants.