Two new representatives of the rare class of bisnorditerpenes, gracilin C and D (2 and 4), have been isolated as minor metabolites from Spongionella gracilis, a Mediterranean sponge. The structure of these compounds, both related to that of gracilin B (3), the major metabolite of S. gracilis, were solved by spectral analyses, including 2D-nmr spectroscopy, and chemical interconversions. Stereostructure 3 has been reassigned to gracilin B instead of the previously proposed 1 on the basis of additional nOe difference experiments and 13C-nmr data.