The Patagonian nudibranch Tyrinna nobilis contains a number of terpenoids, the novel seco-11,12-spongiane tyrinnal (1) and the known sesquiterpenoids dendrolasin (2), pallescensin A (3), and dehydropallescensin-2 (4). The metabolites probably derive from dietary sponges, thus suggesting a parallelism between the ecological relationships of T. nobilis and those of mollusks of genus Cadlina. The structure of 1 was determined by spectroscopic methods.