To investigate the role of tenascin-R in nervous system development, we studied axon pathfinding in the developing optic system of zebrafish. Zebrafish tenascin-R has the same domain structure as tenascin-R in amniotes. Amino acid sequence identity with human tenascin-R is 60%. In 3-d-old larvae, tenascin-R mRNA is expressed in scattered cells throughout the periventricular cell layer of the diencephalon and tectum. Tenascin-R immunoreactivity is not detectable in the optic nerve, optic tract, or tectal optic neuropil but immediately borders the optic tract caudally. Reducing expression of tenascin-R in 3-d-old larvae in vivo by injecting morpholinos into fertilized eggs led to excessive branching of the optic tract in 86% of all injected larvae compared with 20-37% in controls. Branches were almost exclusively caudal, where tenascin-R immunoreactivity normally borders the optic tract, suggesting a role for tenascin-R in guiding optic axons in the ventral diencephalon by a contact-repellent mechanism.