Population responses of single auditory-nerve (AN) fibers to 1-kHz pure tones were investigated in unanesthetized decerebrate cats. Driven rate (spikes/s), Fourier component amplitude (spikes/s), and d'e = (driven rate)/(standard deviation) were examined. These results extend the presently available information. First, the AN population responses, previously observed only in anesthetized animals, are now observed in unanesthetized animals. The anesthetized and unanesthetized animal responses are qualitatively similar. Second, a detectability measure d'e was applied, à la signal detection theory, to populations of AN fibers. A previously unknown behavior of the low-spontaneous-rate (SR) (less than 15 spikes/s) fibers was observed: the d'e increased to large values at moderate and high stimulus levels, giving rise to a large, sharply tuned peak in the d'e-place profile, even when the driven rate was saturated. This behavior was absent in the high-SR (greater than 15 spikes/s) fibers. This observation suggests that the d'e of the low-SR fibers is the best code for stimulus intensity among the different response measures examined, and that the d'e-place profile may be a more precise spatial code for stimulus frequency than the commonly considered rate-place code.