Continuous water temperature measurements were made in a shallow lake in upper Michigan prior to and during ice formation, and during ice growth and decay. Several full circulations of 'overturns' at 4C were observed during autumn and the temperature throughout the lake just prior to complete freeze-over reduced to a minimum of +0.2C. After a permanent ice cover had formed, the water beneath the ice began to warm up. Within a 25-day period during December the water temperature near the bottom of the lake increased from +0.3C to 3.0C. Quantitative analysis of this heat gain showed that it came principally from the energy stored during the summer in the underlying soils. Evaluation of the effect of solar radiation demonstrated that the sun during December contributed very little heat to the water because of the absorption qualities of the cover of snow and bubbly ice. During the ice thaw period in April a unique reduction in temperature was recorded throughout the main mass of water. Since the lake is thermally stratified, it was assumed that this cooling could only result from mechanical action in the water. This phenomenon probably takes place as the surface water from melting snow around the area flows into the lake and causes overturning. (Author).