Between October 2003 and July 2005, aggregation behavior of the sea urchin Astropyga pulvinta Lamarck was studied in Bahía Culebra, Costa Rica. This sea urchin forms aggregations during part of the year and then disappears. I quantified the number of individuals present in a defined area each month, their aggregation behavior between day and night, and their size. Also, temperature and nutrient concentrations of the water were sampled. There were significantly more individuals in aggregations during the colder, upwelling season (December to April). Aggregations consisted of adult individuals that exploit food during the upwelling season. Moreover, these aggregations were used as a refuge by several fish species of high commercial value for the aquarium trade. These sea urchin populations could suffer as extraction of ornamental fishes and urchins increases. Their abundance and behavior should continue to be monitored as an indication of the ecological health of the community.