A condensation nuclei (CN) counter has been developed for balloonborne use at ambient pressures in the troposphere and stratosphere. The instrument employs a thermal gradient diffusion cloud chamber to produce particle growth and a photoelectric particle counter for detection. After extensive laboratory tests, the instrument was successfully flown on several balloon soundings over Laramie. The results show a roughly constant mixing ratio in the stratosphere with a CN concentration of about 20 cm−3 at 15 km. The vertical profile of CN in the troposphere displayed concentration fluctuations ranging between 200 and 2000 cm−3 with a definite maximum in the mixing ratio just below the tropopause.