Five individuals, each from 13 common species of large macroalgae ('macrophytes') from the western Antarctic Peninsula, were surveyed for the presence of filamentous algal endophytes both macroscopically and microscopically using dissecting and compound microscopes. Of the 13 species surveyed, endophytes were either rare or absent in five. The remaining species all supported endophytes in most or usually all individuals with maximum endophyte densities per species ranging from 3% to 75% of the thallus area. Thallus fragments from all individuals with endophytes were placed into culture, and 99 unialgal, filamentous brown algal strains were isolated. The ITS1 gene was sequenced in each strain to sort these into distinct genotypes. Brown algal endophytes grew well in culture, and 10 distinct filamentous genotypes were present. The green endophytes did not grow well in culture, and only two green algal species present in the thallus fragments were isolated. No-choice feeding rate bioassays were performed with thallus fragments of all 13 macrophyte species and with cultures of seven filamentous brown endophytes and both green endophytes. Feeding rates on the endophytes were 2-3 orders of magnitude higher than rates on 12 of the macrophyte species and 2- to 6-fold higher than on the only truly palatable macrophyte, Palmaria deciplens. These data support the hypothesis that Antarctic macrophytes are commonly endophytized and that the endophytes benefit from the association by being protected, at least in part, from amphipod herbivory.