We investigated the natural products chemistry and feeding deterrent effects of brominated sesquiterpenes produced by Pacific collections of the calcareous tropical green algaNeomeris annulata. Assays conducted with whole algae showed thatN. annulata was not susceptible to grazing by natural populations of herbivorous fishes on Guam. Crude extracts and column chromatography fractions containing the brominated sesquiterpenes deterred feeding by herbivorous fishes at natural concentrations in field assays on Guam. Two majorN. annulata sesquiterpenes isolated from Guam collections and three related sesquiterpenes previously reported from Bermuda all deterred fish feeding at the high end of their natural concentration ranges, with the exception of one metabolite from Bermuda collections of the alga that differed structurally from the other compounds. The results support our hypothesis that the compounds produced byNeomeris function as chemical defenses against herbivores. The alga produces both structural defenses (CaCO3 in the form of aragonite) and secondary metabolites that defend against herbivory by reef fishes.