Satellite observations of ozone and chlorine monoxide concentrations during winter 1992-1993 show that in February 1993 chlorine in the lower stratosphere was mostly in chemically reactive forms. Decreases in stratospheric ozone concentration during February and early March 1993 are consistent with chemical destruction by this reactive chlorine. Comparison with changes in the distribution of long-lived chemical and dynamical tracers shows that the observed decrease cannot have been caused solely by dynamical processes.