ABSTRACT. Daily food intake by the herbivorous parrotfish Sparisoma v~nde, as well as assimdation efficiencies of algal food, protein and energy, were quantified through a combination of laboratory feeding tnals and field observations. The intake of algal ash-free dry wt (AFDW) per bite increases hnearly with fish wet wt (FWW, g) and algal biomass (mg AFDW cm-2), and is further determined by the skeletal density of the limestone substrate. Low-density substrates yield higher amounts of AFDW per bite than do high-density substrates. The percentage of the total food intake that is derived from endolithic and crustose coralline algae increases with the size of the flsh, and can be 9 0 % depending on the biomass of epilithic algae. The daily pattern of foraging activity is positively correlated with diurnal changes in food quality, while seasonal daylength variations result in 13 % variation in total daily bites taken. Daily number of bites of S. viride in the field decreases wlth fish size, and is further dependent on life phase and foraging depth. In experiments, fish attained an assimilation efficiency of ca 20% from a natural diet of low algal biomass and high-density dead coral substrates that predominates in the shallow reef. Assimilation efficiency was ca 70 % from a diet of high algal biomass and lowdensity substrates that predominates on the deeper reef parts. In spite of lower daily foraging effort, territorial fish, living in deeper parts of the reef, ingest and assimilate higher amounts of AFDW, protein and energy per day than non-territorial fish foraging on the shallow reef. The difference is caused by increased availability of high-yield food and substrate types inside territories compared to the situation on the shallow reef. Daily assimilated energy (kJ d-') is 0.85 X FWW'.~'~ for fish foraging in the shallow reef zone, and 1.22 X FWW0854 for S. viride foraging inside territories on the deeper reef.