Iridaea cordata (Turner) Bory and Phyllophora antarctica A. et E. S. Gepp (Gigartinales, Rhodophyceae) are common in many high-latitude, shallow benthic marine communities in Antarctica, but previous observations have indicated little or no exploitation by potential herbivores. We have measured retention of test disks over the mouths of the common Antarctic sea urchin Sterechinus neumayeri Meissner, and we have used this assay to examine potential chemical defenses in I. cordata and P. antarctica. Thallus disks of both macroalgal species were retained by the urchins for significantly shorter times than inert filter paper disks and much shorter times than paper disks with a feeding stimulant. Both nonpolar and polar extracts of each macroalgal species added to disks with a feeding stimulant significantly decreased retention times relative to controls. These results support the hypothesis that chemical defenses in I. cordata and P. antarctica play a role in deterring potential herbivores.