Intertidal macroalgae can cope with the dual effects of UV-B irradiation and allelopathy. To study the impacts of the two stressors, we co-cultured Corallina pilulifera with Sargassum thunbergii in 1:1 and 1:10 ratios under different doses of UV-B radiation. The response of the antioxidant defense system, focusing on activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POX) and glutathione reductase (GR), was monitored. In addition, isoenzyme patterns were analyzed using non-denaturing polyacrylamide gel electrophoresis. The results show that the activities of SOD, APX, and GR were all significantly affected by both UV-B radiation and allelopathy, and the effect of their interaction was significant. However, POX activity was only influenced by UV-B radiation. The enzymatic assay revealed four distinct bands of SOD. The SODIII band weakened significantly when the co-cultures were exposed to extremely high dosage of UV-B irradiation under both co-culturing ratios of 1:1 and 1:10. When the co-culturing ratio was 1:10, both POXII and APXII enzyme activities increased with different UV-B doses. GR activity was at its greatest when the co-culture ratio was 1:10 and exposure was to the higher UV-B doses. The activities of GRIII and GRIV were elevated under all UV treatments whereas the activities of GRI and GRII were reduced under the lower UV-B treatments but were elevated under the higher UV-B treatments. However, lipid peroxidation, as indicated by the thiobarbituric acid-reacting substance (TBARS) assay, increased significantly under the dual stressors. Our data suggest that allelopathy and UV-B radiation stress can each affect the antioxidant enzyme activities of C. pilulifera. Critically, the adverse effects of UV-B on C. pilulifera were intensified by the compounding effects of allelopathy. [ABSTRACT FROM AUTHOR]