Symbiotic ectomycorrhizal fungi enhance plant nutrient uptake and growth, especially in nutrient-poor conditions. The changes in plant metabolism due to mycorrhizal colonization could lead to alterations in plant suitability for herbivory. I investigated the effects of these fungal symbionts on oak, Quercus, growth and insect herbivory, and assessed the extent to which herbivore performance was affected by mycorrhizal colonization due to shifts in plant carbon allocation. Northern red oak, Quercus rubra L., acorns were inoculated with the ectomycorrhizal fungus, Pisolithus tinctorius (Persoon), and planted in either a nutrient-rich peat:perlite mixture or a nutrient-poor mine spoil:sand mixture. Plants were grown under greenhouse conditions for 45 d, after which seedling productivity was measured and herbivore performance trials were conducted. Gypsy moth, Lymantria dispar (L.), caterpillars were allowed to feed for the duration of the third instar, and performance was measured. Nonmycorrhizal, nutrient-poor plants had the lowest height growth, and produced the fewest leaves with the lowest leaf area and leaf mass, when compared with plants in the other treatments. Seedlings colonized by fungal symbionts showed enhanced productivity relative to noncolonized seedlings in several respects, including the number of leaves produced, leaf area, and basal stem diameter. Acorns grown in nutrient-rich substrate had a greater number of leaves, leaf area, leaf mass, and leaf area ratio, when compared with acorns grown in nutrient poor substrate, regardless of colonization status. Caterpillar growth and consumption rates were lowest on nutrient-rich, mycorrhizal plants, suggesting that colonized plants decrease herbivore performance in nutrient-rich conditions, when mycorrhizae are functioning parasitically. The data do not support the hypothesis that herbivore performance would increase on nutrient-poor, mycorrhizal plants, when mycorrhizal colonization provides the greatest benefits to the host plant.