Particle size distribution measured from the NASA ER-2 with the new Forward Scattering Spectrometer Probe (FSSP) 300 in a type I polar stratospheric cloud (PSC) on January 24, 1989 show a volume mode near 0.8 &mgr;m diameter. The large increase in particle concentration and volume after cloud entry did not occur until the apparent saturation ratio of nitric acid with respect to nitric acid trihydrate reached 10, but at ratios near 1 subtle changes in the size distribution suggest some type I particles were present. Particle concentrations in cloud of 15 to 17 cm-3 were greater than the CN concentrations of 5 to 7 cm-3 just outside of cloud, suggesting nucleation on more than just sulfate particles. Some particles >4 &mgr;m diameter were observed in a region which was saturated with respect to ice. ¿ American Geophysical Union 1990