The study of the reproduction of Diplodus vulgaris, as part of a base-line study of the fishery resources of the south west coast of Portugal, was based on the analysis of the spawning season, gonad maturation, size-at-maturity, fecundity, and hermaphroditism. The spawning season is relatively long; from December to March, with peaks in January and February. No significant differences were found in the sex ratio (M/F = 1.01) along the year or by sizes. The size at first maturity (L50) for all sexes and undetermined individuals combined was 18.33 cm total length (TL), with no significant difference between males and females. The estimated L50 is considerably greater than the minimum legal size in Portugal of 15.0 cm. Mean absolute fecundity (Fa) was 131,127 oocytes, ranging from 31,523 to 250,608. The relationship between absolute fecundity and total length (TL) (Fa = 25398TL-484426), and with somatic weight (SW) (Fa = 878.8SW-71416) was of the linear type. The mean number of oocytes/g of somatic weight was 526, ranging from 194 to 887. The reproductive strategy of this species is characterised by a rudimentary hermaphroditism with possible protandry, as evidenced by the existence of individuals in sexual transition.