The critical band may be viewed as an indication of the bandwidth of the internal auditory filter. The bandwidth of a filter, as a measure of its frequency selectivity is not easily compared with the bandwidths of other filters. One useful measure of frequency selectivity, however, can be found by dividing the bandwidth of a filter at some arbitrary point below the peak, into the center frequency of the filter. This so called “Q” measure is valuable because it represents a ratio and thus can be compared among filters of different center frequencies. We have assumed that indirect measures of the critical band (the critical ratio) represent the bandwidth of the internal filter at a consistent point below the peak. We have thus calculated a Qcritical ratiofor all the available data points in the parakeet. Psychophysical tuning curves (PTC) have been empirically obtained for eight different probe frequencies in the parakeet. A Q10 dBwas calculated for each of these tuning curves and compared with the data describing Qcritical ratio, at ten different frequencies. The plot over frequency of Qcritical ratioand Q10 dBof the PTC's, although differing in absolute values, were nearly identical in shape. Frequency selectivity is poor in the low frequencies, raises sharply between 2.5 and 4.0 kHz, and then becomes poor again at higher frequencies. The results will be discussed with regard to the relation between the critical band and psychophysical tuning curves and frequency selectivity in the parakeet. [Work supported by NSF.]