To test for threshold effects in the response of coral physiology to increasing seawater flow, field and laboratory experiments were conducted in Moorea. First, the growth of juvenile massive Porites spp. and branching P. irregularis was compared among habitats differing in water motion. Growth of massive Porites spp. responded to flow in a pattern consistent with a threshold effect, whereas growth of P. irregularis increased linearly with flow. Second, a recirculating flume was used to test the effect of flow on photophysiology (DF/Fm', effective photochemical efficiency) for massive Porites spp.; DF/Fm' displayed a threshold response at 23 cm s-1 and 28 °C, but not at 31 °C. Finally, intra-colony variation in the response of DF/Fm' to flow and temperature was explored to evaluate the functional significance of colony shape in small corals. DF/Fm' on the top and upstream surfaces of massive Porites spp. responded with a threshold effect of flow at 28 °C (but not 31 °C), but DF/Fm' on downstream surfaces was unresponsive to flow. DF/Fm' for P. irregularis was less responsive to flow than for massive Porites spp., suggesting that the photophysiological response of corals to varying flow speeds may differ between species and morphologies. Together, these results emphasize that flow can have diverse effects on the physiology of corals, with the outcome depending on flow speed, temperature, location on the colony, and perhaps morphology.