A 41-year-old right-handed male, showing anterior callosal disconnexion symptomes after infarction of the corpus callosum identified by MRI, was studied to elucidate whether each cerebral hemisphere would be specialized even in elementary auditory perception such as hearing pure tone.     The patient was examined for Bekesy audiometry, loudness discrimination and simple reaction time not only with his preferable right hand but also with his left hand. The most noticeable finding in those tests was that the left and right hand produced decisively different results even though the same ear was stimulated.     Bekesy audiometry showed that the thresholds with the right hand were initially higher than the left, but in the subsequent tracings, they conversely kept lower level. This was more remarkable on the contralateral left ear. The tracings with the right hand were far larger in width than the left, and were sometimes suspected to overrun the real hearing limit.     Loudness discrimination test demonstrated the ascending limens of the right hand were lower than the left, however the descending limens of the right hand often reached zero dB increment as if the patient could perceive the change of loudness that had already disappeared.     Auditory and visual simple reaction time of the left hand was shorter than that of the right hand, although the difference was too small to account for the above-described findings.     These results demonstrate that the right and left hemispheres are probably differentiated even in the most elementary auditory perception. The left hemisphere would be less sensitive to the onset of sound, but more active to pursue the sound that had been once perceived, which tends to enlarge perception according to an inner auditory image. The right hemisphere, in contrast, seems to be more passive and naive in receiving the onset and the end of sound.