Abstract Brown algae from tropical and temperate regions of the world's oceans have been shown to contain phlorotannins as typical primary and secondary metabolites. Here, phlorotannin contents of nine abundant Antarctic brown algal species were determined, which ranged between 0.5 and 9% DW. These values were higher than phlorotannin levels in most tropical and North Pacific brown algae but were comparable to levels in Australasian species. Phlorotannins showed clear patterns within distinct thallus portions in most species. Overall, phlorotannin allocation seemed to follow patterns predicted by the optimal defense theory, which states that chemical defenses are allocated preferentially towards thallus parts that are highly valuable to the organism. Phlorotannins also were comprised of distinct size classes in most thallus parts, although trends within individual species should be regarded with care because of the low number of samples per species. Although there were marked differences in size class dis...