Distinct classes of neurons are generated from progenitor cells distributed in characteristic dorsoventral patterns in the developing spinal neural tube. We define restricted neural progenitor populations by the discrete, nonoverlapping expression of Ngn1, Math1, and Mash1. Crossinhibition between these bHLH factors is demonstrated and provides a mechanism for the generation of discrete bHLH expression domains. This precise control of bHLH factor expression is essential for proper neural development since as demonstrated in both loss- and gain-of-function experiments, expression of Math1 or Ngn1 in dorsal progenitor cells determines whether LH2A/B- or dorsal Lim1/2-expressing interneurons will develop. Together, the data suggest that although Math1 and Ngn1 appear to be redundant with respect to neurogenesis, they have distinct functions in specifying neuronal subtype in the dorsal neural tube.