Ground‐level ozone (O3) has been sampled from three environments: La Jolla and Pasadena, California, and White Sands Missile Range, New Mexico, using recently developed techniques for cryogenically collecting and isotopically analyzing samples of atmospheric O3. Significant isotopic variability is observed at each location, in addition to potentially important differences between the sampling locations. The isotopic composition of O3 is a sensitive indicator for the formation and decomposition processes which have influenced the O3 reservoir Thus the isotopic characterization of ground‐level O3 could provide a new source of information regarding atmospheric transformation mechanisms. To date, the measured isotopic variability in ground‐level O3 shows no correlation with O3 or NOx mixing ratios, meteorological parameters, or time of day. However, preliminary results show a correlation between the pattern of isotopic fractionation and degree of photochemical control over the local O3 budget at each sampling location. It should be noted that the conclusions presented are preliminary because of the relatively small data sets, particularly at the Pasadena and New Mexico locations. However, we believe that the results of this research indicate that the isotopic composition of tropospheric O3 is variable and contains information which could be useful in the effort to understand the tropospheric O3 budget.