Critical ratios (CRs) and critical bands (CBs, measured by narrow-band masking) were determined in one and the same test:procedure in 9 housemice (Mus rnusculus). Two spectrum levels of white band-pass noise (0dB, 20dB) and 9 frequencies between 2 kHz and 60 kHz were tested. CBs and CRs follow generally the same frequency-dependent functions. At 2 kHz the width of CRs is significantly larger (p<0.01) than the width of CBs, at 15 kHz CRs are smaller than CBs (p<0.05). At all other frequencies no significant differences (p>0.05) appear. No individual animal tested had significantly larger or smaller CBs (p>0.05) throughout the frequency range than the others. Critical bands represent equidistant parts of about i mm on the basilar membrane of the mouse. The steepness of the slopes of the CB-forming filter is directly proportional to frequency in the range from 10kHz to 50kHz. The critical band related distribution of behaviorally measurable excitation along the basilar membrane of the mouse is shown for the tested range of noise bands and spectrum levels. Present results are in essential agreement with respective data for man.