The microwave emission from the planet Mercury has been measured relative to the flux density of three radio sources at wavelengths of 3.45 and 3 75 cm, using the University of Michigan 85-foot reflector with two different radiometers. The measured mean antenna temperature is 0.05 K. From this value a mean equivalent black-body disk temperature of about 400 K is derived A sub solar point temperature of the planet of 1100 + 300 K (est. m e.) at mean solar distance is derived if it is assumed that the temperature distribution of the sunlit surface of Mercury varies as the one-quarter power of cos 0 where o is the angle of incidence of solar radiation, and that the temperature of the dark hemisphere is zero. A lower sub solar temperature is obtained if radioactive heating and a lunar-type insulating dust layer on the surface are assumed, since the dark hemisphere will then contribute to the mean disk temperature. The effect of orbital libration will also modify the deduced subsolar temperature.