The neural cell adhesion molecule (NCAM) is a cell recognition molecule of the Ig superfamily implicated in cell migration, myelination, and synaptic plasticity, as well as elongation, fasciculation, and pathfinding of axons. Here, we used NCAM-deficient mice to investigate the role of NCAM in the development of the corticospinal tract. We demonstrate severe hypoplasia of the corticospinal tract in adult NCAM mutants. Anterograde tracing of the tract of early postnatal NCAM mutants revealed pronounced pathfinding errors of corticospinal axons. At the pyramidal decussation of mutant mice, some corticospinal axons either stayed ventrally and extended laterally, or axons turned dorsally, but instead of growing to the contralateral dorsal column, a significant fraction of axons projected ipsilaterally. We also observed that corticospinal axons of NCAM mutants entered the pyramidal decussation significantly later than axons of wild-type littermates. Our observations thus demonstrate a critical role of NCAM for the formation of this major axon tract.