A relationship between macroalgae (Phyllophora antarctica and Iridaea cordata), the sea urchin Sterechinus neumayeri, and the sea anemone Isotealia antarctica in Antarctica is described. Both macroalgal species are chemically defended against herbivory by S. neumayeri. Where macroalgae and urchins co-occur in the field, over 95% of the urchins use macroalgae as cover and the vast majority of available drift is held by them. Urchins collected from sites without macroalgae prefer macroalgae over other cover materials in the laboratory, suggesting that they make an active behavioral choice to cover with macroalgae when available. Macroalgal cover acts as a defense against the major sea urchin predator, I. antarctica. Algal cover significantly increases the likelihood that urchins will escape from I. antarctica because the anemones' tentacles attach to the algae, which are subsequently released by the anemone or by both the urchin and the anemone. This defense is physical as thalli from which defensive chemicals have been extracted are equally protective. Macroalgae appear to derive benefit from this relationship because fertile drift plants are retained in the photic zone where they can continue to contribute to the gene pool. The urchins also extend the effective horizontal and vertical distributions of the macroalgae, which may help sustain the range of these algal populations in periods of reduced light availability. Hence, even though the macroalgae are chemically defended from urchin herbivory, this relationship is apparently mutualistic. It benefits the sea urchins by providing a defense against their primary predator and benefits the macroalgae by helping to sustain a reproductive population.