The dichloromethane extract of the brown alga Dictyota dichotoma has yielded a number of known diterpenes together with representatives of two previously unreported carbocyclic diterpene classes. The structure and absolute stereochemistry of the bicyclo[3,3,1]nonane derivative (6) was solved by single-crystal X-ray analysis of a heavy-atom derivative and the gross structure of the dialdehyde (8) was inferred from spectral evidence.