This study tested the effects of acclimatization on the response of corals to elevated temperature, using juvenile massive Porites spp. and branching P. irregularis from Moorea (W149A degrees 50', S17A degrees 30'). During April and May 2006, corals were acclimatized for 15 days to cool (25.7A degrees C) or ambient (27.7A degrees C) temperature, under shaded (352 mu mol photons m(-2) s(-1)) or ambient (554 mu mol photons m(-2) s(-1)) natural light, and then incubated for 7 days at ambient or high temperature (31.1A degrees C), under ambient light (659 mu mol photons m(-2) s(-1)). The response to acclimatization was assessed as biomass, maximum dark-adapted quantum yield of PSII (F (v)/F (m)), and growth, and the effect of the subsequent treatment was assessed as F (v)/F (m) and growth. Relative to the controls (i.e., ambient temperature/ambient light), massive Porites spp. responded to acclimatization through increases in biomass under ambient temperature/shade, and low temperature/ambient light, whereas P. irregularis responded through reduced growth under ambient temperature/shade, and low temperature/ambient light. Acclimatization affected the response to thermal stress for massive Porites spp. (but not P. irregularis), with an interaction between the acclimatization and subsequent treatments for growth. This interaction resulted from a lessening of the negative effects of high temperature after acclimatizing to ambient temperature/shade, but an accentuation of the effect after acclimatizing to low temperature/shade. It is possible that changes in biomass for massive Porites spp. are important in modulating the response to high temperature, with the taxonomic variation in this effect potentially resulting from differences in morphology. These results demonstrate that corals can acclimatize during short exposures to downward excursions in temperature and light, which subsequently affects their response to thermal stress. Moreover, even con-generic taxa differ in this capacity, which could affect coral community structure.