The Sox family of transcription factors are involved in a variety of developmental processes including sex determination and gonadal differentiation. Sox19 is a particularly interesting member of this family that has been found only in fish, though mammals have a very diverged orthologue that is designated Sox15 and assigned to a different Sox family subgroup. Here we describe the cloning and characterisation of sox19 from the European sea bass (Dicentrarchus labrax), an important aquaculture species in which sex ratios skewed in favour of males are frequently encountered. The sea bass sox19 gene contains a single intron, encodes a protein of 309 amino acids, has multiple transcription start sites and may produce a truncated splice variant. Sox19 mRNA is present in many adult tissues, with the highest expression in the brain and gonads. Interestingly, the gene is strongly upregulated in the differentiation of the ovary but not the testis, suggesting a role in ovarian differentiation.