We surveyed the benthic community structure and population density of the long-spined sea urchinDiadema antillarum on the shallow fore-reefs of the Gandoca-Manzanillo Wildlife Refuge, Caribbean Costa Rica, in September and October 2004. In zones with high densities of D. antillarum (>0.6 ind. m–2), the cover of non-calcareous macroalgae, known coral competitors, was low and that of live coral was high, whereas the opposite occurred in zones with low densities of D. antillarum (D. antillarum density was not related to the coverage of calcareous macroalgae, which are not viewed as coral competitors. Mean density of D. antillarum was 0.2 ind. m–2 and the total area covered by live coral was 14%. D. antillarum density and area covered by live coral were 2 and 7 times larger, respectively, than those reported 4 yr earlier for the study site. Within the same period, the proportion contributed by non-calcareous macroalgae to total algal cover declined from ~79 to 48%. Results indicate that various families of scleractinian corals in the Caribbean coast of Costa Rica have increased in abundance, that non-calcareous macroalgae have declined, and that recovering D. antillarum densities are correlated with these observations.