Chromatin remodeling complexes play crucial roles in transcription and are implicated in processes including cell proliferation, differentiation and embryonic patterning. Brg1 is the catalytic subunit of the SWI/SNF chromatin remodeling complex and shows neural-enriched expression. Although early lethality of Brg1-null mice reflects its importance in embryogenesis, this phenotype precluded further study of specific Brg1-dependent developmental processes. Here, we have identified a requirement of Brg1 for both Xenopus primary neurogenesis and neuronal differentiation of mammalian P19 embryonic carcinoma cells. In Xenopus, loss of Brg1 function did not affect neural induction or neural cell fate determination. However, the Sox2-positive, proliferating neural progenitor cell population was expanded, and expression of a terminally differentiated neuronal marker, N-tubulin, was diminished upon loss of Brg1 activity, suggesting that Brg1 is required for neuronal differentiation. The ability of the bHLH transcription factors Ngnr1 and NeuroD to drive neuronal differentiation was also abolished by loss of Brg1 function, indicating that Brg1 is essential for the proneural activities of Ngnr1 and NeuroD. Consistent with this, dominant-negative interference with Brg1 function in P19 cells suppressed neuronal differentiation promoted by NeuroD2, showing the requirement of Brg1 for neuronal differentiation is conserved in mammalian cells. Finally, we discovered that Brg1 physically associates with both Ngnr1 and NeuroD and that interference with Brg1 function blocks Neurogenin3- and NeuroD2-mediated reporter gene transactivation. Together, our results demonstrate that Brg1 (and by inference the SWI/SNF complex) is required for neuronal differentiation by mediating the transcriptional activities of proneural bHLH proteins.