Plant resistance to chilling injury depends on the maintenance of membrane functions, An increase in the degree of unsaturation of membrane fatty acids can help maintain membrane function at low temperatures by reducing the temperature at which deleterious membrane phase-transitions occur. The fatty acid composition of membranes of the chlorophycean Caulerpa prolifera was determined in summer and winter in the Mar Menor coastal Lagoon (SE Spain), In January the percentage of polyunsaturated fatty acids was higher than in August. Membrane content of mono- and diunsaturated fatty acids was similar in the two seasons. The unsaturation index of membranes changed from 76.15 +/- 1.80 (mean +/- standard error) in August to 128.05 +/- 0.32 in January. It is suggested that this increase in the degree of unsaturation might be important to allow C. prolifera to decrease its lower threshold of survival temperature and acclimate net photosynthesis and dark respiration rates to winter temperatures in the Mar Menor.