The architecture of the differentiated nervous system is stable but the molecular mechanisms that are required for stabilization are unknown. We characterized the gene unc-119 in the nematode Caenorhabditis elegans and demonstrate that it is required to stabilize the differentiated structure of the nervous system. In unc-119 mutants, motor neuron commissures are excessively branched in adults. However, live imaging demonstrated that growth cone behavior during extension was fairly normal with the exception that the overall rate of migration was reduced. Later, after development was complete, secondary growth cones sprouted from existing motor neuron axons and cell bodies. These new growth cones extended supernumerary branches to the dorsal nerve cord at the same time the previously formed axons retracted. These defects could be suppressed by expressing the UNC-119 protein after embryonic development; thus demonstrating that UNC-119 is required for the maintenance of the nervous system architecture. Finally, UNC-119 is located in neuron cell bodies and axons and acts cell-autonomously to inhibit axon branching.