Studies are described of the vigilance performance of the cerebral hemispheres of split-brain man. Investigations of visual, auditory and tactual performance are reported for six total commissurotomy patients. The vigilance tasks were lateralized to test the functions of the two hemispheres and make comparison between them. The right hemisphere emerges the superior in its vigilance performance in every case. The results support the association of sustained attention with the functions of the right hemisphere and they suggest that there are different mental mechanisms for selective and sustained attention.