Modeling of past climates is critically dependent on estimates of past sea surface temperatures (SSTs), for which one of the principal techniques used is the measurement of Sr/Ca ratios in corals <Guilderson et al., 1994; McCulloch et al., 1999; Hughen et al., 1999>. The link between coral Sr/Ca and SST is not well-understood and there have been a number of discrepant observations <de Villiers et al., 1995; Alibert, 1998>. Corals with symbiotic zooxanthellae are known to show large diurnal fluctuations in calcification rate associated with the photosynthetic activity of their symbionts. Using detailed measurements with the ion microprobe, we compared the Sr/Ca content of discrete daytime and nighttime skeletal structures in the massive hermatypic coral Porites lutea over the course of 1 year and a seasonal temperature range of 4 ¿C. The Sr/Ca content of daytime skeleton is always lower than that of adjacent nighttime skeleton. While the slope of the nighttime Sr/Ca-SST correlation is close to that seen in inorganic aragonite precipitates, that of the daytime correlation is >4 times as steep. We attribute these differences to the role of photosynthesis in calcification and conclude that bulk Sr/Ca is related principally to daytime calcification rate rather than directly to SST. More reliable estimates of past SST may be arrived at through selective analysis of nighttime skeleton. ¿ 2001 American Geophysical Union