Soft sedimentary material is rapidly accumulating on the bottom of Lake Mead in back of Hoover Dam across the Colorado River. The temperature of this sediment increases sharply with depth in the topmost layers, below which lower temperatures have been recorded with a special penetrating bathythermograph. Bacterial thermogenesis appears to be responsible, at least in part, for the higher temperatures found in the sediment. Millions of living bacteria per gram were found in Lake Mead mud, the largest populations occurring in the topmost layers where the thermocline is steepest. Assuming that the bacteria generate heat at an average rate of 30 x 10 (super -12) calories per cell per hour, ten million might generate 2.6 calories in a gram of mud in a year. Increases of a degree or more in temperature were noted in Lake Mead mud samples incubated for 20 days in thermos bottles, but little or no change occurred in similar mud that had been sterilized by autoclaving. Oxygen and utilizable organic content affect the biochemical heating of mud.