Abstract Ozone is present in much of the United States and has been shown to reduce growth and development of some coniferous tree species. Slash pine ( Pinus elliotti ) is a unique species which occurs naturally only in southern Florida and several of the Lower Keys. The purpose of this research was to determine if mesophyll cells of slash pine needles respond to ozone in a manner similar to that of other conifers, to determine exposure—response relationships of mesophyll cell injury and ozone exposure level, and if needles of native trees that exhibit chlorotic mottle have more mesophyll cell death than asymptomatic needles. Fumigations were performed at four ozone levels with 7-hr mean values of 0, 55, 67, and 71 ppb for 84 days. Results show that ozone injury symptoms occur to mesophyll cells of needles before visible needle injury (chlorotic mottle). The predominant injury to needles was cell necrosis. Ozone exposure—response relationships of exposure (ppm-hr) percentage of dead cells showed a highly statistically significant ( P ) linear relationship of y = 3.56+0.58 ( x ) with a correlation coefficient of 0.86. These results support the conclusion that mesophyll cells of needles of slash pine are very sensitive to ozone. Results presented also indicate that needles from native slash pine trees at Everglades National Park show chlorotic mottle, similar to that exhibited by many conifers, and that such visible injury was generally related to a higher percentage of cell necrosis compared with needles without symptoms.