HSP70 gene expression was studied by quantitative RT-PCR after cloning and sequencing of two different HSP70 gene fragments from the digestive gland of Mytilus galloprovincialis, called MgHSP70 and MgHSC70. Heat shock (1h at 35 degrees C) caused rapid induction of MgHSP70, while no change was observed for MgHSC70. Hg(2+) (150 microg/L for different time periods) significantly induced MgHSP70 expression that reached maximum levels after 24h, decreasing thereafter. MgHSC70 expression was inhibited after 1 day and induced after a 6-day exposure to Hg(2+). A 1-week exposure to Cr(6+) (1, 10, and 50 ng/L) induced and inhibited MgHSC70 and MgHSP70 transcript levels, respectively. MgHSC70 and MgHSP70 appear to play different roles in cell protection; the former is induced after acute stress and/or during the earlier phase of the response while the latter is induced by chronic stress. The present results provide new insights into mechanisms used by mussels to adapt to stressful environments.