To assess the response of mycorrhizae to atmospheric pollutants, 2-year-old northern red oak (Quercus rubra L.) seedlings were grown in natural soils and treated with low levels of O3, SO2, and (or) acidic rain. In a laboratory study, seedlings were exposed to constant concentrations of O3 (0.02, 0.07, or 0.12 ppm) for 7 h per day on 5 days per week and twice per week received 1.25 cm of simulated rain (pH 5.0, 4.0, or 3.0). Ozone treatment caused increases in the number of short roots infected with mycorrhizae per centimetre of lateral root and in the percent mycorrhizal infection. Acidic rain treatments caused decreases in the number of infected short roots and in the percent infection. In a field study, plants were exposed daily in open-top chambers in Ithaca, NY, to air containing 50, 100, or 150% of the ambient O3 concentration. These plants also received either no SO2 or 5 h per day of SO2 (concentration ~0.1 oom) in one-half or one-quarter of the days during the study. Ozone caused increased numbers of infected short roots and decreased percent infection. The significant response of oak mycorrhizae to all three forms of atmospheric pollution suggests that mycorrhizae are a sensitive component of the plant-soil system. Given the importance of mycorrhizae to forest trees, these results indicate a need for more study in this area.