Seasonal variations of some physical, chemical, and biological features were studied from December 1954 to October 1956 in Hot Lake, a shallow saline body of water occupying a former epsom salt excavation in north central Washington. The lake is meromictic and during the period of study had an average salt gradient of approximately 100 g/liter at the surface to 400 g/liter at the bottom, the major salt being magnesium sulfate. The mixolimnion was thin enough that the monimolimnion was heated directly by the sun, with the result that temperatures in excess of 50°C were recorded in the monimolimnion during summer. Much of this heat was retained during the winter. The calculated value of heat gain in the monimolimnion agreed reasonably well with the observed value. The flora consisted mainly of Chara, a bottom mat of blue‐green algae, and a dense population of green sulfur bacteria in the upper part of the monimolimnion. Artemia salina was the dominant zooplankter, and some features of its life history as affected by the unusual thermal properties of the lake are described.