Short-chain fatty acids (SCFAs) were estimated in the gut and arterial blood of tropical marine fishes collected from the vicinity of Lizard Island, Great Barrier Reef Species sampled were predominantly herbivorous, with some related nonherbivorous species. A total of 253 individuals from the following taxa were examined: Acanthuridae, 18 species; Siganidae, eight species; Scaridae, four species; Kyphosidae, one species; and Pomacanthidae, one species. The highest concentration of SCFA in the gut of all species was in the posterior half of the intestine, where the mean acetate concentration per species ranged from 3 to 40 mM. Lower concentrations of propionate, butyrate, and isovalerate were found in the gut of all species. The highest concentrations of SCFAs were found in species that fed on macroalgae, and the lowest concentrations in species that fed on sediments. Substantial concentrations of SCFAs were found in some planktivorous acanthurids. Acetate was present in the blood of species examined at co...