An atmospheric emission line centered at 22.235 MHz has been detected repeatedly over the period of March--August 1975 with a radio telescope located at Westford, Massachusetts (42.5 ¿N, 71.5 ¿W). The line has also been detected in absorption against the sun's microwave continuum. From its exact coincidence in frequency with the well-known 1.3-cm line of the water molecule the line is identified with atmospheric water vapor. A mixing ratio distribution between altitudes of 50 and 80 km is derived from the measured line amplitude and shape. Mixing ratios as large as 15 ppm by volume, about 2 times larger than the maximum predicted by photochemical equilibrium calculations, are found at these mesospheric altitudes.