ABSTRACT: Global climate change and the anthropogenic degradation of tropical reef environments can have deleterious consequences for the health of reef-building corals. Bioindicators of coral status aid in efforts to identify those species andpopulations that are most threatened, which can help focus conservation efforts. The RNA/DNA ratio is an index of protein synthetic capacity and is expected to reflect an organism’s investment in growth. Here, we measured a decrease in the RNA/DNAratio in both the symbiotic anemone Aiptasia pallida exposed to light-deprivation in the laboratory, and in natural populations of the coral Porites astreoides along a depth gradient, suggesting that the RNA/DNA ratio may have depended uponmetabolic activity. Also, RNA/DNA ratios in the coral Montastraea annularis were higher in the winter and spring (when higher growth rates may have been supported) than in summer, at an inshore and an offshore reef in the Florida Keys.Site-specific disparity in bleaching patterns at these 2 reefs may partly explain the differences in their RNA/DNA ratios. Finally, significant interspecific variation was observed in 3 co-occurring species of the genus Montastraea: M.annularis, M. cavernosa and M. faveolata, demonstrating the potential for variability in protein synthetic capacity even between closely related species. These results support the use of the RNA/DNA ratio as an indicator of metabolic activityin natural populations of corals.