Abstract This study tested potential chemical defense of the green seaweed Avrainvillea elliottii against grazing by the sea urchin Lytechinus variegatus. We examined intraspecific variation of defensive chemicals in different parts of the thallus (margin, blade and stipe). We predicted that thallus parts making the greatest contributions to thallus fitness would show the greatest chemical deterrence to herbivory. The susceptibility of the whole alga and its different parts to consumption by L. variegatus was evaluated using assays consisting of powdered algae treated with natural concentrations of crude A. elliottii extract. Results from feeding evaluations showed that the crude extracts of whole plants and different thallus parts deterred consumption by this sea urchin. The results also supported our prediction that stronger chemical defenses occur in the more apical and younger areas of the thallus (margins), parts whose loss would result in the greatest reduction of fitness. These results provide added evidence supporting the ability of siphonaceous or coenocytic green macroalgae to concentrate defensive chemicals in growing parts.