Retinal ganglion cells form orderly topographic connections with the tectum, establishing a continuous neural representation of visual space. Mapping along the dorsal-ventral axis requires interactions between EphB and ephrin-B cell-surface molecules expressed as countergradients in both retina and tectum. We have discovered that the diffusible TGFbeta-related factor Radar (Gdf6a) is necessary and sufficient for activation of dorsal markers, such as Bmp4, Tbx5, Tbx2b, and Ephrin-B2, and suppression of the ventral marker Vax2 in the zebrafish retina. Radar mutant axons innervate only the dorsal half of the tectum, where they form a compressed retinotectal map. Wild-type cells transplanted into the dorsal retina are able to rescue the dorsal identity of nearby mutant cells. Moreover, Radar overexpression "dorsalizes" retinal ganglion cell identity in the ventral retina. We conclude that Radar is near the top of a signaling cascade that establishes dorsal-ventral positional information in the retina and controls the formation of the retinotectal map.