We use airborne lidar measurements of ozone collected during the Texas Air Quality Study (TexAQS) 2000 and TexAQS 2006 field campaigns to compute the horizontal flux of ozone downwind of the Houston and Dallas/Fort Worth metropolitan areas. Fluxes are computed for each aircraft transect by integrating excess ozone (plume ozone minus background ozone) in the urban plumes and multiplying the result by the horizontal wind speed provided by radar wind profilers. In addition, we use the lidar data to estimate ozone production rates and ozone enhancements in the Houston and Dallas/Fort Worth plumes. We found that the average horizontal flux of ozone emanating from the Houston area based on data from six research flights was 3.2 · 1026 molecules per second. This was significantly higher than the flux measured downwind of Dallas/Fort Worth during a single flight. The Houston fluxes exhibited a strong dependence on wind direction. Under southerly or northerly flow, ozone fluxes were about twice as large as under westerly or easterly flow conditions. We estimate that a day's worth of export of ozone from the Houston area could raise regional background ozone by about 10 ppbv over a 40,000 km2 area. This has important ramifications for air quality in communities downwind of Houston as it could raise background ozone levels enough that regions with little or no local pollution sources of their own may violate the federally mandated ozone standard.