The cDNA of Crassostrea gigas HSP70 was cloned and rapid amplification of cDNA (RACE) techniques were used. The full length of HSP70 cDNA was 2045 bp, consisting of a 5' terminal untranslated region (UTR) of 80 bp, a 3' terminal UTR 146 bp, and an open reading frame (ORE) of 1829 bp encoding deduced 620 amino acids. The HSP70 cDNA contained HSP70 family signatures, ATP-GTP binding site motif, tetrapeptide (GGMP) and conserved carboxyl terminal region (EEVD) at C-terminal of deduced amino acid sequence. BLAST analysis revealed that the HSP70 gene has an extreme similarity of 98.9% with C. gigas (AF144646). Northern blotting was used to examine the expression of HSP70 mRNA in the gill tissue of the oyster obtained from surface, middle and bottom layers. The HSP70 mRNA observed the samples taken from middle and bottom layers in September and February, but samples from the surface layer did not find a signal intensity of HSP70 mRNA transcript. Consequently, it seems that the oyster occurring middle and bottom layer have been stressed during the period of summer and winter, which is associated with the massive mortality in Gamak bay.