Apparent depletion of ozone in a cold (0°C), continental stratus cloud system was observed during in situ data collection on 30 April 1994 at the Department of Energy Clouds and Radiation Test Bed site in northern Oklahoma. Analyses of the aircraft data show a significant negative correlation between ozone concentration and liquid water content (LWC) in this cloud. Although droplets of pure water should not significantly affect ozone concentrations, water clouds can potentially perturb ozone through a number of processes, including radiative effects and aqueous-phase reactions in impure cloud droplets. A simple diagnostic model that takes account of cloud effects on the vertical ozone distribution in the boundary layer was constructed to interpret the field data. The results of multifactor regression analysis indicate that aqueous-phase chemistry contributes predominantly to the negative correlation. A depletion of ozone as a function of LWC of about 6.1 ppbv (g m3)1 was found in this particular stratus. In this case, the average in-cloud reduction of ozone is 6% for an average LWC of 0.3 g m3 and ozone mixing ratio of 31 ppbv outside the cloud layer, which is in reasonable agreement with recent model results.