Three in situ, midlatitude, middlay observations of HO2 in the Earth's stratosphere were made using a chemical conversion--resonance fluorescence technique. The mean of the observed HO2 volume mixing ratios at 37, 35, 33, 31 and 29 km are respectively 4.7¿10-10, 2.6¿10-10, 3.9¿10-10, ?2.1¿10-10 and ?1.1¿10-10 with an experimental uncertainty of ¿35%. Significant variability is observed among the three observations. These results are compared with a recent model of stratospheric chemistry demonstrating that while the calculated distribution falls within the band defined by the observations, the mean of the observations lies consistently above the calculation, through never by more than a factor of two. A comparison between the rate of odd oxygen production of O2 photolysis and the rate of HOx catalyzed destruction demonstrates that hydrogen-oxygen free radicals account directly for a fractional destruction of Ox between 5 and 10 percent in the altitude interval between 29 and 37 km given current formulations of stratospheric chemistry.