Weather fleck observed on certain varieties of tobacco since 1952 at the U.S. Department of Agriculture Plant Industry Station, Beltsville, Md., may be indicative of an ozone air pollution problem arising in the Washington, D.C. area. A study of ozone concentrations and meteorological conditions existing prior to and at the time of the occurrence of weather fleck indicates that the high ozone concentrations result from the local formation of ozone due to the photochemical reaction of nitrogen dioxide and certain hydrocarbons in the atmosphere of large cities. This reaction is possibly enhanced by the prior advection of an enriched-ozone tropospheric air mass.