Lidar soundings of the lower stratosphere were made between August 30 and October 11, 1990, at McMurdo Station, Antarctica (78S--167E). Polar stratospheric clouds (PSCs) were observed in only two periods: between September 5 and 10, and on October 7. During these days McMurdo was well within the polar vortex, and temperatures in the lower stratosphere reached seasonal minima. Temperature soundings and two water vapor measurements were also made in the same period. The water vapor content between 11 and 20 km was found to be between 2 and 3 ppmv. Using these values for water vapor, condensation temperatures for water and nitric acid trihydrate were calculated and compared with the temperature measurements. Analysis of these comparisons together with the lidar observations indicated that PSCs appeared only when temperatures were below the threshold condensation point for an air mass containing approximately 1 ppbv niric acid. This was observed from the beginning of measurements on August 30, and suggests that the Antarctic lower stratosphere was highly denitrified both at the end of winter and during the early spring of 1990. ¿ American Geophysical Union 1991