We studied the development of the GABAergic system in the telencephalon of the dogfish Scyliorhinus canicula using GABA and glutamate decarboxylase (GAD) immunocytochemistry. The earliest GABA-expressing cells appeared in the basal telencephalon (subpallium) of stage 24 embryos. Shortly after, the subpallium showed abundant GABA-expressing neuroblasts near the meningeal surface or migrating radially in the neuroepithelium. The limit between the GABA-expressing region and the remainder of the telencephalon (pallium) was sharp and coincides with the pallial/subpallial boundary. At stage 28, GABA-expressing cells with the morphology of tangentially migrating cells (showing a thick growth cone-like leading process) migrate from a dome-shaped protrusion of the lateral subpallium and extended laterally and rostrodorsally into the pallium following either a superficial route or coursing periventricularly. At later stages, abundant GABA-expressing cells were seen in various pallial regions and strings of GABA-expressing cells, possibly migrating, were also noted. The colonization of the dogfish pallium by GABA-expressing cells, originating from the subpallium, is strongly reminiscent of the palliopetal tangential migrations of GABA-expressing cells demonstrated in the telencephalon of mammals and follows similar routes. These results strongly suggest that tangential migrations of GABA-expressing cells appeared very early in vertebrate forebrain evolution.