ABSTRACT: Coral overgrowth by sponge species was studied on Caribbean reefs to determine whether it depends on coral cover and species composition. Overgrowth was quantified in belt transects at 4 localities on the reefs of Curaçao (Netherlands Antilles)and compared to 5 localities sampled on Colombian reefs. Coral perimeter, coral cover, sponge cover, sponge abundance and species richness were measured at each locality. The species richness of aggressive sponges was not influenced by coral cover,whereas the species richness of non aggressive sponges decreased with increasing coral cover. At coral covers of >25%, the sponge community was characterised by more aggressive species. This is a clear example of the importance of spatial competition,indicating that only aggressive sponge species are able to survive on reefs with high coral cover. Below 25% coral cover, the increasing presence of aggressive sponge species resulted in an increasing number of overgrowth interactions. At higher coralcover this relation is distorted. The average coral cover was significantly higher on the reefs of Curaçao than on the Colombian reefs. As a consequence, more sponge species were involved in overgrowth interactions on Curaçao than on Colombian reefs. Theimportance of this overgrowth capacity is emphasised by the impact of sponge species composition. The occurrence of coral overgrowth was dependent on the sponge species composition rather than on the abundance of sponge species. The coral speciescomposition did not influence the frequency of sponge/coral overgrowth interactions. I conclude that competition for space between sponges and corals is important on reefs with high coral cover as more aggressive sponge species and hence more overgrowthinteractions occur. Successful overgrowth of corals by sponges depends on coral cover (irrespective of coral species) and sponge species composition.