Controlled fumigation experiments were conducted to determine the dose-response relationships for four species of urban trees exposed to sulfur dioxide. The species chosen were ginkgo, Norway maple, pin oak, and Chinese elm. Results indicated that resistance to SO2 increased among the species in the following order: Chinese elm, Norway maple, ginkgo, pin oak. Elm showed almost 100% leaf necrosis at exposures over 2 ppm for 6 hr, and severe chlorosis and necrosis at 0.25 ppm for 30 days. Fifty per cent leaf necrosis occurred on Norway maple at 3 ppm for 6 hr, and on ginkgo at 4 ppm for 6 hr, and both species developed moderate marginal chlorosis at 0.50 ppm for 30 days. Injury on pin oak was minor, even at 8 ppm for 8 hr, but at 0.50 ppm for 30 days, a slight overall chlorosis developed on the leaves. The relative susceptibilities of the four species were the same in the long-term as in the short-term exposures. The shapes of the dose-response surfaces indicated that duration of exposure and concentration of the pollutant were of equal importance in producing injury on Chinese elm and probably on pin oak, but on Norway maple and ginkgo, concentration of SO2 was of greater importance than the duration of exposure.