This paper covers the chemical work, recently effected in the authors’ laboratory, on three topics: 1) the defensive behaviour of Mediterranean opistobranchs; 2) marine zoochromes; 3) a new class of alkaloids from a marine sponge. Biosynthetic experiments, performed on the dorid nudibranchs Dendrodoris limbata and Dendrodorisgrandiflora, were aimed at the understanding of the origin and the fate of the harmful molecules used by the molluscs to deter potential predators. New organic compounds were characterized from the opisthobranchs Janolus cristatus and Aglaja depicta. The recent determination of the structure of uranidine (27), the yellow zoochrome of the sponge Verongia aerophoba, stimulated the chemical studies of calliactine, the zoochrome of the sea anemone Calliactis parasitica, and of a pigment co-occurring with hallachrome (29), the red zoochrome of the sea-worm Halla parthenopeia. Some nitrogenous metabolites, belonging to a new class of alkaloids, were isolated from the sponge Reniera sarai and partially characterized, mainly on the basis of a 500 MHz NMR study. © 1986 IUPAC