The detection of microRNAs (miRNAs) at single-cell resolution is important for studying the role of these posttranscriptional regulators. Here, we use a dual-fluorescent green fluorescent protein (GFP)-reporter/monomeric red fluorescent protein (mRFP)-sensor (DFRS) plasmid, injected into zebrafish blastomeres or electroporated into defined tissues of mouse embryos in utero or ex utero, to monitor the dynamics of specific miRNAs in individual live cells. This approach reveals, for example, that in the developing mouse central nervous system, miR-124a is expressed not only in postmitotic neurons but also in neuronal progenitor cells. Collectively, our results demonstrate that acute administration of DFRS plasmids offers an alternative to previous in situ hybridization and transgenic approaches and allows the monitoring of miRNA appearance and disappearance in defined cell lineages during vertebrate development.