A high-sensivity, in situ UV absorption ozone sensor has been developed for use in the stratosphere. The instrument couples 254-nm radiation from a low-pressure mercury discharge lamp into a 40-pass White cell (1200-cm pathlength) to attain a high-sensitivity ozone absorption measurement. Preflight/postflight laboratory tests utilizing an ozone source coupled to a laboratory UV ozone photometer in a fast-flow system as well as in-flight diagnostics verify the successful operation of the instrument. We present evidence to verify that in situ UV absorption ozone photometers can measure stratospheric ozone with better than 3% precision and 5% accuracy, provided proper attention is given to both the thermal field surrounding the gondola and the ambient pressure measurements. We compare our ozone data with modeled profiles in the 28- to 40-km region. An assessment of the disagreement between observations and modeled profiles is given along with suggestions for future experiments designed to constrain photochemical models.