The geochemical compositions of biogenic carbonates are increasingly used for palaeoenvironmental recon- structions. The skeletal δ18O temperature relationship is dependent on water salinity, so many recent studies have focused on the Mg/Ca and Sr/Ca ratios because those ratios in water do not change significantly on short time scales. Thus, those elemental ratios are considered to be good palaeotemperature proxies in many bio- minerals, although their use remains ambiguous in bivalve shells. Here, we present the high-resolution Mg/ Ca ratios of two modern species of juvenile and adult oyster shells, Crassostrea gigas and Ostrea edulis. These specimens were grown in controlled conditions for over one year in two different locations. In situ monthly Mn-marking of the shells has been used for day calibration. The daily Mg/Ca ratios in the shell have been measured with an electron microprobe. The high frequency Mg/Ca variation of all specimens displays good synchronism with lunar cycles, suggesting that tides strongly influence the incorporation of Mg/Ca into the shells. Highly significant correlation coefficients (0.70bRb0.83, pb0.0001) between the Mg/Ca ratios and the seawater temperature are obtained only for juvenile C. gigas samples, while metabolic control of Mg/Ca incorporation and lower shell growth rates preclude the use of the Mg/Ca ratio in adult shells as a pal- aeothermometer. Data from three juvenile C. gigas shells from the two study sites are selected to establish a relationship: T=3.77Mg/Ca+1.88, where T is in °C and Mg/Ca in mmol/mol.