The zebrafish Sox-19 belongs to the Sry subfamily of HMG (Hight Mobility Group) box genes and is closely related to the Sox sub-group B, comprising the mouse Sox-1, Sox-2 and Sox-3 genes, with respect to both HMG box homology (95.3%) and neural expression during embryogenesis. Analysis of Sox-19 expression during embryogenesis by whole-mount in-situ hybridization revealed interesting features. In early gastrula embryos, Sox-19 transcripts are detected within a circular area in the region of the presomptive central nervous system (CNS) and appears to be the earliest molecular marker of the CNS in vertebrates. In the developing brain, ZfSox-19 mRNA is distributed in the ventral region of the diencephalon, midbrain and hindbrain whereas the expression is excluded from the telencephalon. In spite of the ventral localisation of its mRNA, the expression of this ZfSox-19 gene is completely normal in cyclops embryos which implies that ZfSox-19 expression is independent of the presence of the floor plate.