Our previous research suggests that interspecific variation in stress tolerance in intertidal Fucus spp. (Phaeophyceae) is partially mediated by differences in the production of, or ability to detoxify, reactive oxygen. Here we report on the content of antioxidants (ascorbate, glutathione, carotenoids, and tocopherols) and protective enzymes (catalase, superoxide dismutase, ascorbate peroxidase, and glutathione reductase) involved in reactive oxygen metabolism in three species of intertidal brown algae—Fucus spiralis L., F. evanescens C. Ag., and F. distichus L.—that differ in stress tolerance and position in the intertidal zone. Contents of the major antioxidants were similar in the three species and were not correlated with stress tolerance. The least stress tolerant species, F. distichus, had the lowest activity of reactive-oxygen-scavenging enzymes, although F. spiralis, the species with the highest stress tolerance, and F. evanescens contained similar activities of antioxidant enzymes on a fresh-weight basis. However, the activities of superoxide dismutase and ascorbate peroxidase in F. evanescens are lower than those of F. spiralis when expressed on the basis of chlorophyll. These data show that the ratio between reactive oxygen protection and production might be more important than the absolute content of antioxidants and protective enzymes. It also shows the importance of localization of detoxifying mechanisms and avoidance of oxidative stress.