The evolutionary success of reef-building corals in nutrient-poor tropical waters is attributed to endosymbiotic dinoflagellates. The algae release photosynthetic products to the coral animal cells, augment nutrient flux, and enhance the rate of coral calcification. Natural abundance of stable isotopes (delta13C and delta18O) provides answers to modern and paleobiological questions about the effect of photosymbiosis on sources of carbon and oxygen in coral skeletal calcium carbonate. Here we compare 17 species of symbiotic and nonsymbiotic corals to determine whether evidence for photosymbiosis appears in stable isotopes (delta13C and delta15N) of an organic skeletal compartment, the coral skeletal organic matrix (OM). Mean OM delta13C in symbiotic and nonsymbiotic corals was similar (-26.08 per thousand vs. -24.31 per thousand), but mean OM delta15N was significantly depleted in 15N in the former (4.09 per thousand) relative to the latter (12.28 per thousand), indicating an effect of the algae on OM synthesis and revealing OM delta15N as a proxy for photosymbiosis. To answer an important paleobiological question about the origin of photosymbiosis in reef-building corals, we applied this proxy test to a fossil coral (Pachythecalis major) from the Triassic (240 million years ago) in which OM is preserved. Mean OM delta15N was 4.66 per thousand, suggesting that P. major was photosymbiotic. The results show that symbiotic algae augment coral calcification by contributing to the synthesis of skeletal OM and that they may have done so as early as the Triassic.