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Commercially important oysters are often subject to mass mortality in the field. Rationalizing that heat shock can induce tolerance to stresses other than heat, we are exploring the possibility that this treatment might improve subsequent survival in the field. In characterizing the heat shock response in the laboratory, we found that heat shock of 37 C for 1 h (HS) enabled Pacific oysters (Crassostrea gigas) to survive an otherwise lethal heat treatment (43–44 C, 1 h)(LHT). This induced thermotolerance was retained for at least 2 weeks, the longest period examined. Stress proteins of the 70-kDa family were studied in gills by conventional autoradiography and immunoblotting techniques. Two constitutive isoforms of 77 and 72 kDa were up-regulated, and a 69-kDa protein was induced by HS, its synthesis occurring during and after that treatment. Synthesis of these three proteins was reduced to control levels within 2 days after HS, but their amounts remained higher than in control gills for 2 weeks following HS. Immunolocalization of proteins of the heat stock protein–70 (hsp-70) family in histologic sections of gills was consistent with results from Western blotting.