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This study presents the first analysis of the reproductive cycle of the razor clam Ensis arcuatus and the effect of oceanographic conditions. From February 1998 to July 2000, samples were taken at 2-week intervals from the natural submerged bank of the Cies Islands in the Ria de Vigo (NW Spain). A follow-up gonadal condition index (GCI) was conducted and calculated as gonad fresh weight/valve dry weight. From histological preparations of the gonad stained with haematoxylin and eosine, a gametogenic scale was drawn up based on seven stages: (0) sexual rest, (I) start of gametogenesis, (II) advanced gametogenesis, (IIIA) ripe, (IIIB) start of spawning, (IIIC) restoration and (IV) exhaustion. E. arcuatus is gonochoric and the gonadal development of the two sexes is highly synchronous. During the summer, they were in the sexual rest stage, coinciding with the upwelling period in the Ria de Vigo when the cold, nutrient-rich waters from the bottom rise to the surface. This phenomenon produces phytoplanktonic blooms which mean an increase in the availability of food. Gametogenesis began at the start of autumn when upwelling ceased and food was less plentiful. During the winter and spring, consecutive spawns occurred interspersed with gonadal restoration periods.