We sought to determine if the global distribution of sea anemones (cnidarian order Actiniaria) conforms to the classic pattern of biogeography--taxon richness at the equator with attenuation toward the poles--a pattern that is derived almost entirely from data on terrestrial plants and animals. We plotted the empirical distribution of species occurrences in 10° bands of latitude based on published information, then, using the Chao2 statistic, inferred the completeness of that inventory. We found the greatest species richness of sea anemones at 30-40° N and S, with lower numbers at tropical latitudes and the fewest species in polar areas. The Chao2 statistic allowed us to infer that the richness pattern we found is not due to particularly poor knowledge of tropical sea anemones. No 10° band of latitude has less than 60% of the theoretical number of species known, but for only about half of them could we reject the null hypothesis (P = 0.05) that information is complete; anemone diversity is best documented at high latitudes. We infer that the 1089 valid species currently known constitute about 70% of the theoretical total of about 1500 species of Actiniaria. The distribution pattern of sea anemone species resembles that of planktonic foraminiferans and benthic marine algae, although planktonic bacteria, marine bivalves, and shallow and deep scleractinian corals show the terrestrial pattern of equatorial richness attenuating with latitude. Sea anemone species richness is complementary to that of scleractinian corals at many scales; our findings affirm it at the global scale.