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Experiments were performed in Discovery Bay (Jamaica, W I.) to determine the relationship between behavior of threespot damselfish Eupornacentrus planifrons Cuvier, and distribution of the common shallow-water echinoid Diaderna antillarum Philippi. E planifrons actively excludes Diadema from its algal lawn, causing a distinct alteration in local distribution of Diaderna on the reef. Physical attacks on the urchin were usually sustained for 2 5 min. The damselfish's agonistic behavior occurred only dunng the day. Diadema densities were higher withln algal lawns at night than during the day but were always significantly lower than the mean density outside. The size of the damselfish's algal lawn was positively correlated with damselfish density, implying that damselfish which occur in aggregations are able to defend larger territories together than they would when occurring alone. The impact of interactions between fishes, urchins and sessile epibenthos are predictable from experimental data derived from a concurrent study. E. planifrons territories are probably sites of locally high mortality in coral spat, particularly in Agaricia and Porites spp. This would result from competition for space with algae and encrusting epifauna. The algal lawns, however, may also represent refuges for other corals such as Favia fragum which are well-adapted to survive competition with other sessile epifauna as well as with algae. Thus, the algal lawns could represent dlscrete patches In the environment within which mortality of dominant species is high and that of rarer species is relatively low, causing an increase in the latter's representation in the benthic community. The net effect of this would be a n overall increase in coral diversity within the community. This interpretation is consistent with hypotheses concerning the role of patch-formation in determining con~munity structure.