Root biomass, length, and branching frequency, and number and type of mycorrhizal short roots were determined for loblolly pine seedlings grown at two levels of soil Mg and exposed to chronic levels of O3 and simulated acidic rainfall. Seedlings were planted in a sandy loam soil having approximately 15 or 35 mg kg−1 Mg and were exposed to subambient; ambient, or twice ambient concentrations of O3 in open top chambers from May through October. Seedlings also received ambient amounts of simulated rainfall at pH 3.8 or 5.2. Root biomass, length, and branching frequency were not significantly affected by O3, rainfall acidity, or soil Mg treatments. Seedlings grown in the subambient O3 treatment had a greater number of short roots infected with mycorrhizae than seedlings grown in ambient or twice ambient O3 treatments, but trends were not statistically significant. Increasing rainfall acidity and soil Mg concentration resulted in a significantly (P = 0.07) greater number of mycorrhizal short roots, due primarily to an increased occurrence of one corraloid mycorrhizal type. Results suggest that mycorrhizal fungi differ in their response to O3, rainfall acidity, and soil Mg status, and suggest that mycorrhizal infection is more sensitive than seedling root growth to O3, acidic rainfall, and soil Mg status.