Normal animal development requires accurate cell divisions, not only in the early stages of rapid embryonic cleavages, but also in later developmental stages. The Caenorhabditis elegans unc-85 gene is implicated only in cell divisions that occur post-embryonically, primarily in terminal neuronal lineages. Variable post-embryonic cell division failures in ventral cord motoneuron precursors result in uncoordinated locomotion of unc-85 mutant larvae by the second larval stage. These neuroblast cell division failures often result in unequally sized daughter nuclei, and sometimes in nuclear fusions. Using a combination of conventional mapping techniques and microarray analysis, we cloned the unc-85 gene, and find that it encodes one of two C. elegans homologs of the yeast Anti-silencing function 1 (Asf1) histone chaperone. The unc-85 gene is expressed in replicating cells throughout development, and the protein is localized in nuclei. Examination of null mutants confirms that embryonic neuroblast cell divisions occur normally, but post-embryonic neuroblast cell divisions fail. Analysis of the DNA content of the mutant neurons indicates that defective replication in post-embryonic neuroblasts gives rise to ventral cord neurons with an average DNA content of approximately 2.5 n. We conclude that UNC-85 functions in post-embryonic DNA replication in ventral cord motor neuron precursors.