In the Drosophila central nervous system, cellular diversity is generated through the asymmetric partitioning of cell fate determinants at cell division. Neural precursors (or neuroblasts) divide in a stem cell lineage to generate a series of ganglion mother cells, each of which divides once to produce a pair of postmitotic neurons or glial cells. An exception to this rule is the MP2 neuroblast, which divides only once to generate two neurons. We screened for genes expressed in the MP2 neuroblast and its progeny as a means of identifying the factors that specify cell fate in the MP2 lineage. We identified a P-element insertion line that expresses the reporter gene, tau-beta-galactosidase, in the MP2 precursor and its progeny, the vMP2 and dMP2 neurons. The transposon disrupts the neurogenic gene, mastermind, but does not lead to neural hyperplasia. However, the vMP2 neuron is transformed into its sibling cell, dMP2. By contrast, expression of a dominant activated form of the Notch receptor in the MP2 lineage transforms dMP2 to vMP2. Notch signalling requires Mastermind, suggesting that Mastermind acts downstream of Notch to determine the vMP2 cell fate. We show that Mastermind plays a similar role in the neurons derived from ganglion mother cells 1-1a and 4-2a, where it specifies the pCC and RP2sib fates, respectively. This suggests that Notch signalling through Mastermind plays a wider role in specifying neuronal identity in the Drosophila central nervous system.