A summary of the results of 10 balloon-borne in situ measurements of ClO in the stratosphere is presented. Alll observations were carried out close to local noon at 32¿N latitude with solar zenith angles between 24¿ and 55¿. The mean of nine of the observations, which for the most part fall within a well-defined envelope, is compared with a recently calculated distribution between 25 and 42 km at the appropriate latitude and time of day. It is found that while reasonable agreement exists in the 35- to 40-km region, the ratio of calculated to observed increases monotonically with decreasing altitude below 35 km, reaching approximately three at 25 kn. The tenth observation exhibits a mixing ratio nearly an order of magnitude greater than the mean. An analysis of the relevant chemistry below 30 km is presented which indicates that the discrepancy between the observed mean and the calculated distribution can be linked to uncertainties in the / ratio in the lower stratosphere. A comparison is made between the in situ resonance fluorescence observation reported here and the remote laser heterodyne radiaometer measurements of Menzies. Finally, the fractional destruction of Ox = O3+O(3P) by ClOx = Cl+ClO, based on the observed mean of the in situ and 3P)> midday observations, is presented for the altitude interval 27--41 km.