Although previous studies have implied that interneurons continue to migrate into the corpus callosum (CC) during the postnatal period, little is known about the details of the migration pattern. In this study, we analyzed the entire postnatal CC every other day and showed the presence of a transient handlebar-like structure traversing the CC during the second postnatal week. Using immunostaining and two strains of transgenic mice, we showed that this handlebar-like structure consisted of GABAergic interneurons derived from the caudal ganglionic eminence, which expressed the transcription factor Sp8. Data obtained from in-utero electroporation experiments showed that these high-density interneurons were adjacent to callosal axons. Thus, we provide the first direct evidence that a transient Sp8+ interneuron handlebar-like structure exists in the CC during a brief postnatal critical period. Copyright (C) 2015 Wolters Kluwer Health, Inc. All rights reserved.