The topography and trajectories of the commissural fibers of the superior temporal region (STR) are studied using the autoradiographic technique. The superior temporal region is connected with the opposite cerebral hemisphere by way of two commissures. The rostral third of the supratemporal plane (STP) and superior temporal gyrus (STG) sends commissural connections through the anterior commissure. The caudal portions of the STP and STG, including the primary auditory area, send their interhemispheric connections via the caudal corpus callosum only. The mid-portion of the STR sends interhemispheric fibers through both the corpus callosum and anterior commissure. In the mid-sagittal plane, interhemispheric fibers coursing in the anterior commissure are located in its ventral portions and those fibers coursing through the corpus callosum are located in its caudal portion rostral to the splenium. It appears that this pattern of interhemispheric connections of the STR is related to the architectonic characteristics of the areas of origin of the fibers. The rostral STR, which has less well-differentiated cortical lamination patterns, sends fibers via the anterior commissure while the posterior STR fibers, coming from more differentiated cortices, travel by way of the corpus callosum.