The central nervous system (CNS) represents an excellent model system for examining how a multitude of unique cell fates are specified. We find that asymmetric localization of the numb protein autonomously controls a binary cell fate decision in the Drosophila CNS. The simplest lineage in the Drosophila CNS is that of the MP2 precursor: it divides unequally to generate the dMP2 and vMP2 neurons. Both are interneurons but project in different directions: dMP2 projects its axon posteriorly while vMP2 projects anteriorly. During MP2 mitosis, numb is localized into dMP2 and excluded from vMP2. Loss of numb transforms dMP2 into vMP2, whereas ectopic numb produces the opposite transformation of vMP2 into dMP2. Thus, numb is asymmetrically localized in the dividing MP2 and is necessary and sufficient to autonomously specify dMP2 neuronal identity.