Frequency analysis is a fundamental function of the auditory system. Békésy and Katsuki believed that sharpening of frequency tuning by lateral inhibition takes place in the central auditory system. However, most 'cat' auditory physiologists have believed that frequency tuning of neurons is not sharpened in the central auditory system, so that there is no lateral inhibition. Unlike quasi-triangular frequency-tuning curves of peripheral neurons, pencil- or spindle-shaped frequency-tuning curves have been found in the central auditory systems of many species of animals belonging in different classes. Inhibitory tuning curves are commonly associated with such 'level-tolerant' sharp excitatory tuning curves. It is that frequency-tuning curves of some central auditory neurons are sharpened by inhibition. Yasuji Katsuki (Professor, M.D., Ph.D.) passed away on 6 March 1994 at the age of 88. I have written this article as a tribute to him, focusing on his major contribution to auditory neurophysiology: the finding of the sharpening of frequency tuning in the cat's central auditory system. Neural sharpening of frequency tuning is an old yet still current topic, as you will read in this article dedicated to Professor Katsuki.