The response to cold of liver and heart membrane lipid composition and mitochondrial respiration in reared sea bass (Dicentrarchus labrax L.) was investigated. Fish acclimation was followed during the natural seasonal cycle from August to March. The data on the fatty acid composition of liver and heart polar lipids and on total lipids of liver mitochondria and microsomes did not indicate any increase in unsaturation in response to cold. The enzyme complexes of the liver and heart mitochondrial respiratory chain showed a repeated negative compensation for cold acclimation. The constancy of the break in the Arrhenius plot of liver cytochrome oxidase (EC 22.214.171.124) was consistent with the lack of homeoviscous adaptation of membrane lipids. A thermoadaptive strategy based on the reduction of sea bass metabolic activity is suggested.