Glyphosate-resistant (GR) horseweed (Conyza canadensis L. Cronq.) now dominates horseweed populations in the eastern San Joaquin Valley (SJV) of California. This is an agricultural area with high levels of ground level ozone and considerable application of glyphosate for vegetation management. In chamber studies with vegetative seedlings we have found that the SJV biotype of GR is more robust than the glyphosate-sensitive (GS) biotype in the absence of both ozone and glyphosate. This advantage declines with increasing ozone exposure. The GR biotype develops earlier than GS, accounting in part for increased biomass in the rosette stage. GS and GR exhibited similar sensitivity to increasing ozone in the absence of glyphosate. While this sensitivity to ozone was similar between the biotypes in the presence of glyphosate, the effect of the combined stresses of ozone and glyphosate caused the GS biotype became non-viable and it was preferentially removed from the population. Thus high ozone conditions may help to drive the species toward a pure glyphosate resistant population. As the California GR biotype exhibits greater earlier robustness than GS under California conditions, this may partially explain the recently observed increased aggressiveness of horseweed in this environment.