The Mytilus gonadal cycle in Vigo Estuary follows the histological pattern proposed by Lubet (1957, 1959). We show details not previously described, due to sexual differences, environmental characteristics and high periodicity samplings: an out of phase between reserve tissue and gonadal cycles; a “bridge stage” determined by intermediate characteristics among the last spawnings, gonad resorption and regeneration of the reserve tissue; a moving forward of the initial stages and different duration by sexes; and clear-cut differences between spawnings and restorings: in winter, they are a slow and less intense process, with abundant reserve tissue, high atresia and they do not end in effective spawnings, whereas in spring are very rapid and intense processes causing massive spawnings. The atresia present two phases: cytoplasmatic structures self-lysis and digestion by hemocytes. Winter atresia is related to unfavorable conditions to stimulate spawning. Temperature and food availability determine the temporal evolution of the gametogenic cycle in male and female differently. The mantle tissue index varies depending on the number of gametes and is suitable as a gonadal index in this species.