The UNC-119 proteins, found in all metazoans examined, are highly conserved at both the sequence and functional levels. In the invertebrates Caenorhabditis elegans and Drosophila melanogaster, unc-119 genes are expressed pan-neurally. Loss of function of the unc-119 gene in C. elegans results in a disorganized neural architecture and paralysis. The function of UNC-119 proteins has been conserved throughout evolution, as transgenic expression of the human UNC119 gene in C. elegans unc-119 mutants restores a wild-type phenotype. However, the nature of the conserved molecular function of UNC-119 proteins is poorly understood. Although unc-119 genes are expressed throughout the nervous system of the worm and fly, the analysis of these genes in vertebrates has focused on their function in the photoreceptor cells of the retina. Here we report the characterization of an unc-119 homolog in the zebrafish. The Unc119 protein is expressed in various neural tissues in the developing zebrafish embryo and larva. Morpholino oligonucleotide (MO)-mediated knockdown of Unc119 protein results in a "curly tail down" phenotype. Examination of neural patterning demonstrates that these "curly tail down" zebrafish experience a constellation of neuronal defects similar to those seen in C. elegans unc-119 mutants: missing or misplaced cell bodies, process defasciculation, axon pathfinding errors, and aberrant axonal branching. These findings suggest that UNC-119 proteins may play an important role in the development and/or function of the vertebrate nervous system.