The sea urchins Paracentrotus lividus and Arbacia lixula were collected monthly for 1 y in the southern coast of Spain, and their gonad fatty acids were analyzed to evaluate seasonal variations. The gonad fatty acids of A. lixula significantly differed from those of P. lividus, probably because of substantial differences in diet. In both sea urchins, seasonal changes were mainly characterized by the marked decrease of the major polyunsaturated fatty acid 20:5n-3 in summer and early autumn, and the opposite increase of 20:1n-11, 18:1n-9 and the principal nonmethylene-interrupted diene 20:2Δ5,11. Arachidonic acid showed the lowest levels in winter, increasing in April through May, and remaining at relatively high levels until the end of the year. Saturated fatty acids were less affected by seasonality, although they tended to decrease with advancing maturity of the gonad and to increase after spawning. These seasonal changes are discussed in relation to the annual reproductive cycle and to fluctuations in seawater temperature.