European sea bass, Dicentrarchus labrax L., is an important commercial species along the Atlantic and Mediterranean coastline in both fishing and aquacultura. The artificial propagation of this species may lead to reduced amounts of genetic diversity. Although many biological topics related to the aquculture of this species have been studied (see Barnabe & Billard 1984), information on the amount and distribution of genetic variation throughout the range of the species is scarce (Benharrat, Pasteur, Siau & Bouian 1985: Cervelli 1985: Martínez, McEwen, McAndrew & Álvarez 1991: Child 1992; Patarnello, Bargelloni, Caldara & Colombo 1993; DeLeon, Chikhi & Bonhomme 1997). In the present study, the investigation of the genetic resources of the sea bass along the Spanish littoral, initiated by Martínez et al. (1991), was extended to a survey of a cultivated Mediterranean population. Allozymes were analysed and the variations were estimated. Any detectable change in genetic structure, potentially caused by rearing conditions, was assessed and compared to previously analysed populations in the Atlantic and Cantrabric.