Satb2 is a DNA-binding protein that regulates chromatin organization and gene expression. In the developing brain, Satb2 is expressed in cortical neurons that extend axons across the corpus callosum. To assess the role of Satb2 in neurons, we analyzed mice in which the Satb2 locus was disrupted by insertion of a LacZ gene. In mutant mice, beta-galactosidase-labeled axons are absent from the corpus callosum and instead descend along the corticospinal tract. Satb2 mutant neurons acquire expression of Ctip2, a transcription factor that is necessary and sufficient for the extension of subcortical projections by cortical neurons. Conversely, ectopic expression of Satb2 in neural stem cells markedly decreases Ctip2 expression. Finally, we find that Satb2 binds directly to regulatory regions of Ctip2 and induces changes in chromatin structure. These data suggest that Satb2 functions as a repressor of Ctip2 and regulatory determinant of corticocortical connections in the developing cerebral cortex.