The agonistic behavior of the sea urchin Echinometra lucunter (L.) was studied in the algal ridge reefs of Boiler Bay, St. Croix, U.S. Virgin Islands, where it inhabits burrows in the reef surface. Sixty-four encounters were set up by placing an urchin (intruder) at the opening of an occupied burrow (host). In 46 cases, agonistic behavior between the host and intruder resulted. Most of the interactions involved pushing, but biting was frequently observed. The encounter usually resulted in the eviction of the intruder from the burrow, but occasionally the host was evicted or the urchins cohabited the burrow. Burrow occupants successfully evicted all intruding urchins of their own or slightly larger size, and regained possession of their burrows after being experimentally replaced by an urchin of equal size. As Echinometra live in dense aggregations in wave-swept areas, this behavior may be significant in defending protected living space and access to a food supply.