The basic helix-loop-helix transcription factor NeuroD1 is expressed in embryonic and adult mouse olfactory epithelium (OE), as well as during epithelial regeneration, suggesting that it plays an important role in olfactory neurogenesis. We characterized NEUROD1-expressing progenitors, determined their progeny in the adult OE, and identified a subtle phenotype in ΔNeuroD1-knockout mice. All olfactory sensory neurons (OSNs) derive from a NeuroD1-expressing progenitor as shown by recombination-mediated lineage tracing, as do other sensory receptors of the nose, including vomeronasal, nasal septal, and Grunenberg ganglion neurons. NEUROD1-expressing cells are found among the globose basal cell population: they are actively proliferating and frequently coexpress Neurog1, but not the transit amplifying cell marker MASH1, nor the neuronal marker NCAM. As a consequence, NEUROD1-expressing globose basal cells are best classified as immediate neuronal precursors. In adolescent ΔNeuroD1-LacZ knock-in null mice the OE displays subtle abnormalities, as compared to wildtype and heterozygous littermates. In some areas of the OE, mature neurons are absent, or sparse, although those same areas retain immature OSNs and LacZ-expressing progenitors, albeit both of these populations are smaller than expected. Our results support the conclusion that most, if not all, nasal chemosensory neurons derive from NeuroD1-expressing globose basal cells of the immediate neuronal precursor variety. Moreover, elimination of NeuroD1 by gene knockout, while it does not disrupt initial OSN differentiation, does compromise the integrity of parts of the olfactory epithelium by altering proliferation, neuronal differentiation, or neuronal survival there. J. Comp. Neurol. 519:3580–3596, 2011. © 2011 Wiley-Liss, Inc.