Marine sponges of the genus Mycale are a source of novel bioactive nitrogenous metabolites; e.g., mycalysins A and B,2 mycalamides A and B,3 and mycalolides A-C. In our continuing search for cytotoxic metabolites from Japanese marine invertebrates, we found that the lipophilic extract of the marine sponge Mycale adhaerens showed potent cytotoxic activity. Bioassay-guided isolation yielded five active compounds, of which 13-deoxytedanolide and a brominated isocoumarin named hiburipyranone are new. This paper deals with the isolation and structure elucidation of these compounds.