Seasonal variations in the gonad index and the biochemical composition of gonad and gut tissue of the sea urchin, Paracentrotus lividus (Echinodermata: Echinoidea) were determined. Samples were collected from Lorbé (Galicia, north-western Spain) between November 1994 and October 1995. The gametogenic cycle of the sea urchin was annual, with a long, single spawning period from May to August 1995. Most of the biochemical components in gonad showed clear seasonal variation, which was related to the gonad cycle. The main component was protein, which ranged between 36 and 60% of the dry weight, with maximum levels coinciding with the minimum levels of glycogen. Gonad glycogen therefore seemed to be the energy source employed for protein synthesis during gametogenesis. Seasonal variation in total lipids in the gonad was less evident; the main lipidic component was triacylglycerol (around 10% of the dry weight), whereas total cholesterol accounted for less than 1%. The gut does not seem to act as a storage tissue in P. lividus and seasonal variation in biochemical components may be a consequence of fluctuations in the availability of food.