Concurrent measurements of crystal number densities and concentrations of absorbing material contained in cirrus crystals reveal statistically higher crystal number densities in areas of the cloud where absorbing material exceeds 0.01 μg m-3. This enhancement in crystal number density ranged between a factor 1.6 and 2.8 depending on the aerosol number density. The absorption signature appeared as banded structures 2 to 8 km wide, which suggests aged aircraft plumes as the source of absorbing material. In the altitudes of the most frequently used flight levels, the concentration of absorbing material in crystals exceeded 0.01 μg m-3 during more than 40% of the time in cirrus clouds. ¿ 1998 American Geophysical Union