Mussels (Mytilus galloprovincialis) are the main bivalves cultured in the Mediterranean Sea and in the Thermaikos gulf North of Greece. Global warming might affect these cultures since increases in the sea temperatures especially during summertime and may cause mass mortalities. Thus projections of the effects of global warming and the knowledge of the mechanisms of thermal limitation and adaptation are considered important in this context. In the present work we studied the seasonal expression of Hsp70 and Hsp90 of aquacultured mussels in nature in an effort to examine whether mussels are seasonally thermally stressed. Moreover we determined the activity of the key glycolytic enzyme PK in order to examine seasonal changes in the metabolic profile. The present data showed a biphasic seasonal pattern in the expression of Hsps in the mantle and PAM of M. galloprovincialis. The expression of inducible Hsp72 and Hsp90 increased between February and mid April, and remained constant or decreased slightly by the end of May. Hsps expression started again in early June leading to a gradual increase by mid July and remained by the end of August. The pattern of the HSR indicates increased protein turnover or protein damage in the tissues of aquacultured M. galloprovincialis between winter and summer seasons. The induction temperature is probably limited, and beyond some upper limits there is no further change in the HSR induction temperatures and the HSR sets in. PK activity in the mantle was significantly higher in winter than in spring. Early May saw a reactivation of PK to higher than spring values. Similar to mantle PK, PK activity in PAM was significantly higher in winter than in spring. A slight reactivation was observed by the end of August. © 2009 Elsevier B.V. All rights reserved.