The nucleation rates for submicron-diameter nitric acid trihydrate (NAT) aerosol particles were measured under stratospheric conditions using a temperature-programed flow tube and Fourier transform infrared extinction spectroscopy to detect the phase change from liquid to solid. The temperature range from 175 to 155 K was examined. The measured nucleation rate constants (in units of cm-3 s-1) are between 3.8¿1.8¿1010 and 9.7¿6.3¿1012 for the temperature range between 167.2 and 163.5 K. The experiments show directly that critical nuclei form rapidly in NAT droplets at these temperatures, but subsequent crystallization is very slow, so the nucleated droplets can persist as liquids for long times unless their temperature is raised to increase the crystal growth rate. In these experiments, complete crystallization of the nucleated droplets was achieved in the observation time (about 10 s) by raising their temperature to near 180 K. Although demonstrable in the laboratory, the freezing of NAT by homogeneous nucleation at low temperature and subsequent warming are unlikely to be important for type 1 polar stratospheric cloud formation because the temperatures required for nucleation are lower than those commonly found in the lower stratosphere. ¿ 1998 American Geophysical Union