In the developing CNS, neurons and glia are sequentially produced from the ventricular neural progenitor cells. One fundamental question in developmental neurobiology is what signals or factors control the developmental switch from neurogenesis to gliogenesis. Here we report that microRNAs (miRNAs) play an essential role in this important developmental process. Inhibition of miRNA formation in Olig1(Cre)-mediated Dicer conditional knock-out mice disrupted both oligodendrogenesis and astrogliogenesis in the ventral neuroepithelial cells. By contrast, the early patterning and development of motor neurons were not affected in the mutant spinal cord tissue.