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Food selection by the Caribbean stoplight parrotfish Sparisoma viride was investigated on a fringing coral reef of Bonaire, Netherlands Antilles. For different reef zones, the diet composition for each life phase was determined by description of randomly selected bites, and compared to the availability of food resources, as determined with the aid of chain-link transects. S. viride employs an excavating grazing mode, and feeds almost exclusively on algae associated with dead coral substrates. Preferred food types are large and sparse turfs growing on carbonate substrates inhabited by endolithic algae. Crustose corallines, with or without algal turfs, are not preferred. Feeding forays were longer on the preferred food types. Foraging preferences are related to nutritional quality of the food types and their yield, i.e. the amounts of biomass, protein and energy that can be ingested per bite, as calculated from the size of grazing scars and the biochemical composition of the algae. In spite of selective foraging, a large proportion of bites is taken on inferior food types. Endolithic algae constitute an important food resource for scraping herbivores, such as S. viride, These algae have relatively high energetic value, and allow a high yield as a result of weakening the carbonate matrix by their boring filaments. The yield of algal resources also depends on the skeletal density of the limestone substrates. On deeper reef parts (> 3.5 m depth), low-density substrates predominate, resulting in higher yields of algae per bite than are attained from high-density substrates that predominate on shallower reef parts. The increased availability of high-yield food and substrate types coincides with the occurrence of haremic territorial behaviour in S. viride males on the deeper reef parts. Territories are defended against conspecifics and have an important function as spawning sites. It is argued that the access to superior food resources on the deeper reef makes territorial defence feasible for S. viride.