Airborne lidar measurements have been made of polar stratospheric clouds (PSCs) during the Airborne Arctic Stratospheric Expedition in January--February 1989. These show the existence of a systematic relationship between the backscatter depolarization ratio, Δ, and the (aerosol+molecular)/molecular backscatter ratio, R. The data are consistent with a two population PSC particle model. The first population consists of particles with low values for Δ (≤0.02) and low to intermediate values for R (≤6). The second population is characterized by higher values for Δ (up to 0.60) and R (up to 40 or greater). The relationship between Δ and R, observed within a limited range of altitudes and latitudes, corresponds to a mixture of the two populations with variable concentrations but constant optical properties. ¿ American Geophysical Union 1990