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The thermal regime of Lake Michigan is described on the basis of the analysis of 1,671 bathythermograph casts made in 1954 and 1955. The beginning, duration, geographic extent, and ending of thermal stratification were determined from 51 thermal profiles from all areas of the open lake. The lake gained heat for about 5 months (mid-March to mid-August) and lost heat over the rest of the year. It was thermally stratified during much of the warming cycle and vertically isothermal during much of the cooling cycle. The annual warming period began by April and was well underway by mid-May. During the transitional period between the isothermal conditions of winter and the thermal stratification of summer, the rate of warming was relatively fast. Seasonal warming began in the shallow inshore waters, which were usually thermally stratified by mid to late May. Stratification persisted in the deeper offshore water into December. Maximum surface temperatures and highest average metalimnion temperatures were reached in early August. Local variations in water temperatures during stratification, however, were a common feature of the thermal characteristics of Lake Michigan. These sometimes sudden changes were caused by upwellings of cold bottom waters that in some instances affected areas of hundreds of square miles. During the warming period of 1954, the heat content of Lake Michigan increased an average of 425 gm-cal/cm??/day. The annual heat budget of the lake for 1954 was 51,700 calories which is similar to that reported for 1942 (50,000 calories) and 1943 (62,000 calories). Heat was lost from mid-August until March, at a rate of about 250 gm-cal/cm??/day. Seasonal thermal characteristics differed little between 1954 and 1955, and were similar to those observed during studies in 1941 and 1942.