This study presents in situ measurements performed in two cumulonimbus anvils over western Europe during the Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM) in July 1994. Sampling of cloud particles between 5 and 60 μm aerodynamic diameter was performed using a counterflow virtual impactor. Peak values of cloud water content and crystal number density for this size-range exceeded 2 g m-3 and 200 cm-3 (normalized to standard temperature and pressure), respectively. Concurrent measurements of aerosol particles, water vapor, CO, and O3 support the idea that much of the air in the anvil has been rapidly transported directly from the boundary layer while experiencing limited dilution. ¿ 1999 American Geophysical Union