ABSTRACT: Chemical defenses that are rapidly activated in response to injury have been reported in numerous species of vascular plants, but activated chemical defense has been demonstrated for only 1 genus of seaweed. To investigate the frequency ofpotential activated chemical defenses in seaweeds and to determine if there are geographic differences in the frequency of theses, we conducted urchin and fish feeding assays using chemical extracts from 42 species of temperate or tropical seaweeds thatwere damaged immediately before extraction in organic solvents (= the potentially activated extract) or placed in organic solvents before they were damaged (= the non-activated extract). Seven species exhibited changes in palatability consistent withactivated defenses while 4 species became more, rather than less, palatable if they were damaged 30 s before extraction. Frequency of activation did not vary geographically. Seventeen percent of tropical species (4 of 24) and 17% of temperate species (3of 18) exhibited changes in palatability that were consistent with activation of chemical defenses. Thin-layer chromatography of lipid-soluble extracts indicated that damaging the thallus prior to extraction caused noticeable chemical changes in 70% ofthe species evaluated. Investigations of algal chemical defenses thus need to consider the effects of injury during herbivore attacks and the effects of extraction methodology on the types of, and concentrations of, metabolites discovered in seaweeds.