Environmental variations can be recorded in bivalve shells, and this proxy information can be crucial for paleoceanographic studies in regions where other proxies (e.g. corals) are absent. We present the first sclerochronological study of Protothaca thaca (Mollusca, Veneridae), a common upper subtidal bivalve, characterized by a high temperature tolerance, from the SE Pacific Ocean (12 to 45 degrees S). P. thaca is present in archaeological deposits and Holocene marine terraces, making it a potentially useful paleo-environmental recorder, particularly in respect to El Nino Southern Oscillation (ENSO) reconstructions. The sclerochronological analysis of recent P. thaca shells from Peru and Chile was done by microscopy of thin sections and acetate peel replicates; it shows that a clear 'summer check' (SC) is formed in the shell when sea surface temperature (SST) exceeds 18 degrees C for several weeks. Under La Nina conditions in summer, SCs can be absent due to lower SST and enhanced productivity. During the strong 1997-1998 El Nino, long-lasting high summer SST produced an SC > 520 mu m wide. The relative width of SC on fossil shells may thus be an indicator of paleo-ENSO events. Our sclerochronological analysis has established a temporal framework in P. thaca shells, providing precise time-series analytical profiles, which is a prerequisite for future calibration of geochemical proxies.