Studies were performed to determine the injury and growth response of four native tree species to ambient ozone on ridgetop sites of the Allegheny Plateau of north central Pennsylvania. Open-top chambers were established at each of three sites located in Clear Creek State Park, Jefferson County (41°9′39″N, 79°02′35″W), Elliott State Park, Clearfield County (41°07′02″N, 78°31′40″W), and Tiadaghton, Lycoming County (41°20′05″N, 77°26′57″W). Seedlings of black cherry (Prunusserotina Ehrh.), yellow-poplar (Liriodendrontulipifera L.), red oak (Quercusrubra L.), and red maple (Acerrubrum L.) were exposed to ambient air or to charcoal-filtered air containing approximately 95, 60, or 40% of ambient ozone in randomized, complete blocks in 1988 and 1989. Black cherry seedlings were also exposed in 1990. Ozone was greatest in 1988 at all sites, and total ozone during the 3-year study was greatest at the westernmost sites (Clear Creek and Elliott). Foliar stipple injury of black cherry and yellow-poplar were correlat...