The temperature structure of Post Pond, a small (46.6 hectares), mid-latitude, dimictic lake in west-central New Hampshire, was studied during autumn, winter and spring of 1968-1969. The lake was instrumented over its maximum depth (11.7 m) with a string of 24 thermocouples which recorded hourly temperatures. Temperatures in 9 m of sediments underlying the lake were measured with a thermistor probe. Secondary and tertiary thermocline development in the epilimnion occurred during short warming periods in the early autumn. The autumn overturn lasted 25 days, whereas the spring overturn lasted only 4 days. The entire lake mixed isothermally in the autumn to 3.2C. During the period of ice cover, the lower 5 m of water gained approximately 51.5 cal/sq cm, which was supplied by stored heat in the bottom sediments. A steady-state thermal gradient of 0.07C/m was found for the deeper sediments underlying the lake during ice cover. Late winter cooling of bottom water under the ice cover may be the result of snowmelt in areas adjacent to the lake causing activation of groundwater influx. Melting of the clear ice portion of the ice cover was primarily the result of heat supplied to the lake from snowmelt water, and occurred on the underside of the ice sheet. Thermal instability of the water mass persisted for 9 days during peak snowmelt runoff; this can be partially explained by an increase in dissolved solids with depth. (Author).