Measurements of cirrus clouds and aerosol in the lower stratosphere and upper troposphere have been performed with a mobile lidar system aboard the German research vessel Polarstern. The aerosol load in the lower stratosphere is higher in the tropics compared to the midlatitudes. Our data reveal a high frequency of occurrence of subvisual cirrus clouds in the tropical tropopause region. Based on the data of radiosondes the temperature of the clouds as well as the structure of the tropical tropopause is characterized. The mean optical depth was 0.02 and a mean temperature of 198 K was determined. Cloud tops are often found at the thermal tropopause. In some cases these clouds are extemely thin with an optical depth below 10-3.