Possible ecological roles of antibacterial, antifungal, antiviral, cytotoxic and antimitotic activities found in western Mediterranean benthos were investigated, and relationships were sought between these activities and taxonomic groups, presence of fouling organisms, and community structure. Cytotoxic and antimitotic activities are the most abundant, and are widespread in almost all the taxonomic groups studied. Porifera, Bryozoa and Tunicata contain the most biologically active chemicals. Cytotoxic molecules are more frequently present in tunicates than in bryozoans. There is a close association between antimitotic and cytotoxic, as well as between antibacterial and antifungal, activities. As antifouling defences, cytotoxic and antimitotic activities seem to be less effective than antibacterial and antifungal ones; the latter appear to function in a generalist antifouling mode. Chemically rich species are much more abundant in sciaphilic/cryptic habitats than in photophilic ones.