The structure of larger-scale motions in Lake Ontario, which are under the influence of the earth's rotation, is analyzed by the application of (1) rotary spectrum analysis, (2) 'constant phase' shape or 'harmonic lock' representation, and (3) bispectrum analysis. A set of continuous wind, current, and temperature data at several depths was extracted from the records of the International Field Year for the Great Lakes network. The results indicate that apart from the well-known primary spectral peak near the inertial period there exists a significant secondary peak at half this period. A shift of the primary peak beyond the limiting inertial period is observed near the steepest vertical temperature gradient at stations closer to shore. It is suggested that the secondary peak is due to resonant interaction of spectral components near the inertial frequency and that the observed frequency shift below the inertial frequency is due to interaction between the low-frequency waves of the Kelvin type and the Poincar¿ type waves near the inertial frequency.