Resonant frequencies of inner (IHC) and outer (OHC) hair cell systems in the guinea pig cochlea were computed using data on sensory-hair stiffness obtained from in vitro organ of Corti preparations (D. Strelioff and Å. Flock (1984): Hearing Res. 15, 19–28). IHC stereocilia were modelled as stiff, free-standing, uniform cylinders which rotate about their elastic attachments to the apical surfaces of IHC. OHC, with the overlying tectorial membrane (TM), were modelled as a resonant mechanical system with the TM providing mass and the three rows of OHC sensory-hair bundles providing elasticity for linear, simple harmonic motion parallel to the reticular lamina. Since computed IHC resonant frequencies increase from 128 kHz at the apex to 300 kHz at the base, it is unlikely that they contribute to frequency selectivity. In contrast, computed frequencies of the OHC-TM system are within the audio range, increasing from 1.2 kHz at the apex to 22 kHz at the base. The results of these computations support the hypothesis that the OHC-TM system contributes to mechanical frequency selectivity of the cochlea whereas IHC are passive receptors which respond to mechanical movements of the cochlear partition.