Pacific oysters, Crassostrea gigas, living at a range of tidal heights, routinely encounter large seasonal fluctuations in temperature. We demonstrate that the thermal limits of oysters are relatively plastic, and that these limits are correlated with changes in the expression of one family of heat-shock proteins (HSP70). Oysters were cultured in the intertidal zone, at two tidal heights, and monitored for changes in expression of cognate (HSC) and inducible (HSP) heat-shock proteins during the progression from spring through winter. We found that the "control" levels (i.e., prior to laboratory heat shock) of HSC77 and HSC72 are positively correlated with increases in ambient temperature and were significantly higher in August than in January. The elevated level of HSCs during the summer was associated with moderate, 2-3 degrees C, increases in the upper thermal limits for survival. We measured concomitant increases in the threshold temperatures (T(on)) required for induction of HSP70. Total hsp70 mRNA expression reflected the seasonal changes in the expression of inducible but not cognate members of the HSP70 family of proteins. A potential cost of increased T(on) in the summer is that there was no extension of the upper thermal limits for survival (i.e., induction of thermotolerance) after sublethal heat shock at temperatures that were sufficient to induce thermotolerance during the winter months.