The homeobox gene sine oculis (so) is essential for visual system formation in Drosophila. A vertebrate member of the so/Six gene family, Six3, is expressed in the developing eye and forebrain. Injection of Six3 RNA into medaka fish embryos causes ectopic Pax6 and Rx2 expression in midbrain and cerebellum, resulting in the formation of ectopic retinal primordia. Injected mouse Six3 RNA initiates ectopic expression of endogenous medaka Six3, uncovering a feedback control ofSix3 expression. Initiation of ectopic retina formation reveals a pivotal role for Six3 in vertebrate retina development and hints at a conserved regulatory network underlying vertebrate and invertebrate eye development.