Measurements by monostatic lidar of linear depolarization ratios and backscatter coefficients in an altostratus cloud revealed a horizontally layered structure. Three different types of layers were observed. The bottom and central layers had depolarization ratios varying from 0.3 to 0.4, which identified them as layers containing mainly ice. The backscatter coefficients were similar to those found in cirrus ice clouds. A central, transient layer had depolarization ratios characteristic of a high-density water cloud, although the total integrated backscatter of 2.3 ± 1.2 was high for this type of cloud. The top layer had a depolarization ratio of 0.2 at the cloud base, decreasing to 0.04 at the cloud center. Backscatter coefficients ranged up to 30 km−1 and the total integrated backscatter was about 7.6 ± 3.8. This value is considerably higher than the range of values predicted for water or cirrus ice clouds and one possible explanation is that specular reflection was occurring from horizontally aligned ice crystal plates. The variation of backscatter coefficient within each layer was rather regular, with a maximum at the center of the layers. The cloud was situated in a stable air stream and its evolution appeared to be slow.