ABSTRACT: Sets of artificial reefs were replicated in 5 bays off Tortola in the British Virgin Islands to investigate the effects of habitat complexity on fish assemblages. Increasing percentage hard substrate and the number of small reef holes increasedfish abundance on reefs. The observed number of species (S_obs) occurring on each reef increased with increasing rugosity, variety of growth forms, percentage hard substrate, and variety of refuge hole sizes. A rarefied or abundance-correctedspecies richness measure (S_rare) was calculated to take the varying fish abundances into account. After this correction, rugosity was the only variable that significantly increased fish species-richness. Experimental reefs of different height(20 and 60 cm) did not have significantly different fish abundance or species richness. The presence of long-spined sea urchins Diadema antillarum increased S_obs and total fish abundance on artificial rock-reefs and in seagrass beds, butthe effect was most pronounced in seagrass beds where shelter was a strongly limiting factor. These results indicate that complex habitats or animals such as D. antillarum that provide shelter to fish are essential for maintaining fish biodiversityat local scales. The most important aspects of complexity are rugosity, hard substrate and small refuge holes. Artificial reefs may be used to mitigate habitat damage in impacted areas, and if management objectives are to increase local fish abundance andspecies richness, the reefs should provide a stable substrate where this is unavailable, have a rugose surface with many small refuge holes, and have a variety of growth forms.