Single unit recordings were made from auditory nerve fibers in the monkey to pure tone stimuli. Extensive studies of frequency response areas suggested the existence of two types of auditory nerve fibers. One type showed a flat-bottomed response area and the other showed a conventional sharp one. On the other hand, curves plotting the discharge rate versus the intensity of the sound stimulus at various frequencies indicated two clear tendencies. In crossed ramp type fibers, the curve around the characteristic frequency (CF) showed a slow inclination, while the other curves showed a steep increment. In parallel ramp type fibers, the curves showed the same steep tendency. By the above two criteria, a total of 118 units could be divided into 40 and 78 unit groups which seemed to correspond to the outer spiral fiber and the inner radial fiber respectively. The entire distribution of Q(10) of the auditory fibers was the sum of the two log-normal distributions with different natures where Q(10) = CF/bandwidth at 10 dB above threshold.