Water vapor in the earth's mesosphere has been observed at the frequency of 22.235 Ghz as an absorption against the sun by utilizing a ground-based radio telescope. The H2O mixing ratio of 4.3¿1 ppm is obtained in the height 40-70 km for a constant distribution model. The data are equally well represented by a photochemical model of Crutzen (1974), which was scaled by a factor of 0.93 yielding a peak mixing ratio of 6¿1 ppm at 55 km. These results are 3-day averages during November 1979, where the averaging periods cover the about ¿4 hours of the sun's transit of our meridian. The measurements are insensitive to H2O above 70 km, where the absorption is very weak. We cannot estimate the mixing ratios below 50 km, since the pressure broadening in the line exceeds the bandwidth of our spectrometer at the lower altitudes.