Relative ozone sensitivity was evaluated according to morphological and histological characteristics in four Pinus species grown in containers under greenhouse conditions in California's San Bernardino National Forest. The secondary leaves of the more ozone-sensitive species exhibited a larger number of stomata per cross-sectional area of mesophyll cells while the number of mesophyll cells per stoma was lower. The number of hypodermal layers and thickness of epidermal and hypodermal layers were negatively correlated with ozone sensitivity. In more ozone-tolerant pine species, initial injury was more closely associated with substomatal mesophyll cells. Ozone injury affected a greater proportion of the entire needle of the sensitive species as evidenced by histological symptoms closer to the needle base.