Reconstructing the past ENSO (El Niño Southern Oscillation) interannual variability requires very high resolution sea surface temperature (SST) proxies. In order to compensate for the lack of coral archive in the Eastern Pacific cold tongue, we studied the potentiality of the bivalve species Mesodesma donacium as a SST tracer for the Peruvian coast. Biomineralisation cycles and growth line formation in the shells are shown to be controlled by tides. The sclerochronology based on fortnightly cycles allowed us to compare local SST data with high resolution shell records. As a result of the high correlation between SST and aragonite δ18O, we calculated a linear SST-fractionation relationship for the 14-20°C range: T(°C)=(17.41±1.15)-(3.66±0.16)*(δ 18Oarag./PDB-δ18Owat./SMOW). The difference from the commonly used Grossman and Ku (1986) [Grossman, E.L., Ku, T.L., 1986. Oxygen and carbon fractionation in biogenic aragonite: temperature effect. Chemical Geology 59, 59-74] equation suggests the potential existence of a species dependence of vital effect within bivalves. This preliminary equation allows monthly-scale SST reconstructions with a ± 1.5°C precision using M. donacium archaeological shells from southern Peru, giving potential insights into the problem of past ENSO behaviour. © 2005 Elsevier B.V. All rights reserved.