MicroRNAs (miRNAs) belong to a conserved class of small non-coding RNAs that are typically 18-25 nucleotides long. They are found in both animals and plants. These small RNAs can regulate gene expression at translational level by interacting with their target messenger RNAs, and they play an essential role in the development of plants and animals. To date, more than 200 miRNAs have been identified in mammals; however, their mRNA targets have not yet been identified. In this study, we demonstrate that the expression of Hes-1, which is a basic helix-loop-helix transcriptional repressor, is regulated by miRNA-124 in P19 cells. Reduction in the levels of miR-124 mediated by locked nucleic acids resulted in the accumulation of Hes-1 and hindered the retinoic acid-induced neuronal differentiation of P19 cells. Thus, our results indicate that miR-124 regulates the expression of Hes-1 at the post-transcriptional level and is involved in the retinoic acid-induced neuronal differentiation of P19 cells.