High-altitude tropical cirrus (TC) have been observed by lidar during a cruise of the German research vessel 'Polarstern' above the Atlantic between 8¿S and 12¿. The backscatter lidar data give a detailed picture of the vertical structure of the clouds with a high time resolution. Using the data of radiosondes, which were launched twice a day aboard 'Polarstern,' the temperature of the clouds as well as the structure of the tropical tropopause is characterized. The clouds are found in an altitude between 14 km and 17 km, with a mean optical depth of τ = 0.02 and a mean temperature of 198 K. Tropical Cirrus appear in two distinct layers which are separated by a change in wind direction. This structure was observed south as well as north of the ITCZ. Cloud tops are often found at the thermal tropopause, in some case these clouds are of the ultra-thin type.