Symbiotic dinoflagellates belonging to the genus Symbiodinium (zooxanthellae) play an important role in ecological specialization and physiological adaptation in corals, We examined the diversity and depth distribution of zooxanthellae in 5 morphospecies of Madracis at the Buoy I study-reef on Curacao, Netherlands Antilles, Following earlier studies, we examined length and sequence variation in the D1 and D2 domains of the nuclear rDNA, large subunit (LSU) of Madracis-associated zooxanthellae. Both RFLP and sequence comparisons showed that all 5 Madracis morphospecies host a single type of Symbiodinium belonging to phylogenetic Group B sensu Rowan. No correlation was found between zooxanthellae and habitat depth, The presence of the single, Type-B zooxanthellae in all Madracis morphospecies at Buoy I (and from 3 other biogeographic locations in the Caribbean) suggests that 'generalist' zooxanthellae-coral associations are equally successful over a range of habitats and that adaptations to different light and nutrient regimes are not necessarily dependent on the mix of zooxanthellae types or zonation with depth, as has been shown in the well-studied Montastraea annularis complex and Acropora cervicornis. A review of the current literature on zooxanthellae diversity in scleractinians (including biogeographic sampling for some species) shows that most species appear to harbor only 1 zooxanthellae type and that the 3 types of Symbiodinium (A, B, C sensu Rowan) are found at all depths and are thus potentially always available for acquisition.