Heat shock proteins are a multigene family of polypeptides composed of the constitutively-expressed heat shock cognate (HSC) members and of the stress-inducible (HSP) proteins, whose expression is specifically induced by stress factors. Constitutive and inducible 70 kDa isoforms are reported in vertebrates and invertebrates. HSCs are expressed in all bivalve molluscs studied to date, while the occurrence of strictly heat-inducible HSPs seems a distinctive feature of oysters. To gain more insight into the molecular features of the Ostrea edulis HSP70, we have cloned and sequenced the gene product putatively encoding for the heat-inducible isoform HSP69 and examined the pattern of expression after heat exposure. Four different clones of approximately 1794 bp were obtained that share a high degree of homology with heat-inducible HSP70 from other bivalves. Amino acid sequence comparisons indicated that the main structural features of the heat-inducible HSP70 are highly conserved within the O. edulis HSP70 clones, while lower sequence homology occurred with respect to HSC70 transcripts. Northern blot analysis indicated that HSP69 mRNA was absent in control animals but induced after heat shock (1 h at 32 degrees C or higher). Induction was detectable immediately after heat shock, reaching a maximum after 2 to 3 h of post-stress recovery at 18 degrees C, and decreasing thereafter. A phylogenetic analysis of the HSP70 family members from oyster and other bivalves revealed a substantial conservation in the evolutionary pattern among constitutive and inducible gene products, from invertebrates to higher vertebrates.