N-[2-(2-oxo-l-imidazolidinyl)ethyl]-N’-phenylurea, (EDU or ethylenediurea) reduced the sensitivity of petunia plants (Petunia hybrida Vilm.) to ozone when treated with foliar or root applications. For most plants, EDU gave protection from visible injury within 24 hours of its application as a spray or soil drench. It was much more effective than butanedioic acid mono-(2,2-dimethylhydrazide) (daminozide) as a protectant from ozone injury. The activity of EDU was unaffected by the addition of a surfactant to the spray solution or soil drench, by day length, light conditions, temperature, and physiological preconditioning of the plants. Protection from injury by ozone lasted for at least 14 days. Furthermore, a concentration of 500 ppm either as a foliar spray of 1 to 2 ml per plant or as a soil drench of 100 ml per 10-cm diameter pot protected an ozone sensitive cultivar, ‘Snow Magic,’ the moderately sensitive cultivars, ‘White’ and ‘Pink Cascade,’ and the least sensitive cultivars, ‘Comanche Improved’ and ‘Sugar Plum’. Treatment with EDU afforded no protection against sulfur dioxide exposure doses which produced acute injury.