Rates of ammonia excretion, and respiration to excretion (atomic O:N) ratios were measured for three species of scleractinian coral from the Bahamas, during August 1986 and March 1987, to test the hypothesis that zooxanthellate reef species have lower rates of amino acid catabolism and higher dependence on lipid and carbohydrate catabolism than nonzooxanthellate species. Freshly collected individuals of two reef species,Montastrea annularis andAcropora cervicornis, have significantly lower mean ammonia excretion rates [51 ± 66 nmol (mg-at N)−1 h−1 and 192 ± 172 nmol (mg-at N)−1 h−1, respectively] than those of the tropical nonzooxanthellate speciesTubastrea coccinea [257 ± 68 nmol (mg-at N)−1 h−1]. The temperate nonzooxanthellate coralAstrangia poculata has mean excretion rates [632 ± 242 nmol (mg-at N)−1 h−1] which are much higher than those of all three tropical species. O:N ratios for the two reef species were generally greater than 300, while those of the nonzooxanthellate species ranged from 17 to 39 forT. coccinea and from 8 to 12 forA. poculata. The two reef species conserve nitrogen by having relatively low rates of amino acid catabolism, and support most of their metabolic needs by catabolizing the lipids and carbohydrates they receive from their zooxanthellae. The tropical nonzooxanthellate species has lower rates of ammonia excretion and respiration, and higher O:N ratios than the temperate nonzooxanthellate coral, which may be an indication that the former has less food available to it. The ammonia production rates of the reef species would support doubling times (growth rates) of the zooxanthellae of 13 to 22 d. These growth rates agree with observed rates obtained by mitotic index methods, and with suggestions that growth of zooxanthellae within many corals may be nitrogen-limited.