In February--March 1999, nocturnal observations with a ground-based backscatter/depolarization lidar were conducted during the Airborne Platform for Earth Observation--Third European Stratospheric Experiment on Ozone (APE-THESEO) campaign at Mah¿, Seychelles (4.4¿S, 55.3¿E). Upper tropospheric cirrus clouds were a common feature in the lidar echoes; they were detected for about 67 hours out of a total measurement time of 125 hours. Optical and geometrical characteristics of the clouds detected at altitudes above 9 km throughout the campaign are derived and discussed. Cirri above 14.5 km present a layered structure and appear more persistent and thinner than cirri at lower altitudes. The behavior of average optical and geometrical depths, integrated depolarization, backscatter-to-extinction ratio, integrated backscatter, and extinction coefficient is studied as a function of midcloud temperature and is compared with results from previous measurements and models. The estimated cloud backscatter-to-extinction ratio ranges from 0.01 to 0.2 sr-1, with an average value of 0.051 ¿ 0.030 sr-1. In the temperature range between 200 and 240 K the integrated depolarization ranges between 0.27 and 0.19 and appears lower than that for midlatitude cirri. In a single night, very high values of the integrated depolarization for a layered cirrus at 16 km could be related to outflow from an active convective cell. The two highest and coldest (temperature around 188 K) layers show values of optical and geometrical depth and integrated depolarization lower than for cirri at temperatures between 190 and 200 K. The optical characteristics of the clouds decrease for temperatures from 190 to 200 K, possibly indicating different evolution phases of the clouds.