The midline of the fruit fly and the zebrafish share more than just topology. Fricke et al. show that zebrafish use a receptor encoded by the gene astray for guiding axons from the developing eyes to the brain, crossing the midline on their way to form the optic chiasma. The astray receptor is similar to the receptor encoded by the gene roundabout that is responsible for guidance of growing axons across the midline in Drosophila. Chimeras made by exchanging eyes of mutant and wild-type zebrafish demonstrated that the relevant site of expression of astray is in the eye.