An acoustic echo sounder (called sodar) has been developed, designed and fabricated. This equipment has been operating successfully for several years and has been shown to have important applications in the areas of air pollution control/studies, microwave and millimetre wave radio communication including troposcatter, aviation and micrometeorology. Work on the development of the sodar was taken up at the National Physical Laboratory (npl) in 1972. At that time, this was one of the few equipment of its kind in the world. It has been designed in the laboratory and has been built with indigenous components. The equipment currently probes the thermal structure of the atmosphere upto a height of 700 m but can be extended to a kilometre. Two sodars are in operation-one at the National Physical Laboratory, Delhi and the other at the Micrometeorological Laboratory, Tarapur in collaboration with barc, Bombay. The design criteria of the npl sodar and experiments with it are described in this paper. We believe that the technique will soon become an important tool in the areas of micrometeorology, radio communication, aviation and air pollution studies.