Periods and structure of gravitational free oscillations in Lake Huron, Saginaw Bay, Georgian Bay, and the North Channel are determined from theoretical calculations. The calculations take into account the bathymetry and shape of the Huron system and also the effect of the earth's rotation. Time series analyses are performed on water level data from 13 stations around the lake. The power and coherence spectra exhibit peaks corresponding to diurnal and semidiurnal forcing and varous modes of free oscillation of the lake. Both theory and observations show that Saginaw Bay, Georgian Bay, and the North Channel each have a fundamental frequency of oscillation lower than the frequency of Lake Huron's first longitudinal oscillation. There is reasonable agreement theoretical and observed characteristics for several of the free modes fo the Lake Huron system.