The urchin Diadema antillarum Philippi (Echinodermata: Echinoidea) was extremely rare to absent from reefs throughout Jamaica during much of the 1980s and part of the 1990s following a Caribbean wide mortality event. This survey was conducted in 2007 to assess their distribution on the western forereef of Discovery Bay, Jamaica. Mean urchin densities of 3.93 ± 3.4 m-2 (mean ± SD) are the highest that have been recorded in Discovery Bay since the mortality event. The increases in D. antillarum numbers of the past decade have been accompanied by reductions in macroalgal cover. This study found a strong inverse correlation (Pearson product-moment correlation = -0.597, p <0.000) between macroalgal cover and Diadema density, indicating that the urchins are exerting a strong top-down control on macroalgae. Any future increases in urchin densities will most likely result in further reductions of macroalgal cover.