We used the teleost Astyanaxmexicanus to examine the role of the lens in optic nerve and tectum development. This speciesis unusually suited for studies of nervous system development and evolution because of its two extant forms: an eyed surface dwelling (surface fish) and several blind cave dwelling (cavefish) forms. Cavefish embryos initially form eye primordia, but the lens eventually dies by apoptosis, then the retina ceases to grow, and finally the degenerating eyes sink into the orbits. Transplantation of an embryonic surface fish lens into a cavefish optic cup restores eye development. We show here that retinal nerve fibers are formed and project to the optic tectum in cavefish embryos. In adult cavefish that have completed lens degeneration, however, the number of retinal axons in the optic nerve is substantially reduced compared to surface fish. The presumptive brain domains of embryonic cavefish are not altered relative to surface fish based on expression of the regional marker genes Pax6, Pax2.1, and engrailed2. In contrast, the adult cavefish brain is elongated, the optic tectum is diminished in volume, and the number of tectal neurons is reduced relative to surface fish. Unilateral transplantation of an embryonic surface fish lens into a cavefish optic cup increases the size of the optic nerve, the number of retinotectal projections from the restored eye, and the volume and neuronal content of the contralateral optic tectum. The results suggest that the lens has a specific influence on optic nerve and tectum development during eye growth in Astyanax.