The mesoscale analysis of ozone concentrations measured at ground level and aloft in the Boston metropolitan area has provided some insight into the origin and fate of urban ozone. A period of intensive airborne ambient air monitoring, conducted by the U.S. Environmental Protection Agency's Environmental Monitoring and Support Laboratory-Las Vegas long-range air-monitoring aircraft from August 9 through August 14, 1975, shows areas of semipersistent high and low ozone concentrations. In addition, data presented identify an urban ozone plume at extended distances downwind of Boston on several days within this sampling period. The importance of the daily synoptic meteorological situation upon the observed ozone distribution is emphasized. On August 9, ozone concentrations ranging as high as 103 ppb were measured aloft 35 km downwind of Boston. On this same day, immediately upwind of Boston, a surface ozone ridge was evident. Data collected on August 10–12 show that with persistent wind directions aloft an urban plume is discernible at distances of 50–100 km downwind. Ozone values as high as 148 ppb were measured within the urban plume; in fact, ozone concentrations exceeded 80 ppb as far as 200 km over the ocean, downwind of Boston, on August 12. On August 11 and 13 the local weather, situation apparently caused a sea breeze related surface ozone ridge pattern to recirculate high ozone levels over the Boston area late in the evening. August 14, a non-sea breeze day, featured a cold front passage through the Boston area. The ozone distribution pattern at the surface differed from surface patterns for other days studied. The ozone distribution pattern aloft was also less complicated than patterns aloft on other days studied.