The hippocampus has a highly ordered structure and is composed of distinct layers. Neuronal migration is an essential part of the process of the layer formation because neurons are primarily generated near the ventricle and must migrate to arrive at their final locations during brain development. Impairment of brain development is thought to underlie the etiology of psychiatric disorders. Consistent with this idea, many genetic risk factors for psychiatric disorders play critical roles during brain development. As one example, Disrupted-in-Schizophrenia-1 (DISC1) is a genetic risk factor for major psychiatric disorders and plays various roles during neurodevelopment. To examine the role of Disc1 in the hippocampal development, we suppressed expression of Disc1 in the CA1 region of the developing mouse hippocampus by using the RNA interference (RNAi) technology and an in utero electroporation system. Disc1 suppression was found to impair migration of the CA1 pyramidal neurons. This effect was especially apparent while the majority of the transfected neurons were passing through the stratum pyramidale of the developing hippocampus. The migration of neurons was restored by expression of an RNAi-resistant wild-type mouse Disc1, indicating that the migration defect was caused by specific suppression of Disc1. In the mature hippocampus, the migration defect resulted in malposition and disarray of the pyramidal neurons. These findings indicate that Disc1 is required for migration and layer formation by the CA1 pyramidal neurons during hippocampal development.