In an initial experiment, inoculation of sour orange (Citrus aurantium L.) seedlings in sand with Glomus fasciculatus greatly increased growth with or without fertilization with P or Cu. Application of P or Cu to non-mycorrhizal seedlings in sand did not stimulate growth. Application of P to non-mycorrhizal seedlings in a sandy loam soil increased growth, but induced Cu deficiency symptoms and reduced foliar Cu concentrations. Application of P to mycorrhizal seedlings did not induce Cu deficiency symptoms, but reduced foliar Cu concentrations slightly. In a complete factorial experiment, sour orange seedlings in the sandy loam soil, were inoculated with G. fasciculatus or not inoculated, fertilized with 5 levels of P from 0 to 800 mg P 1-1 of soil and 3 levels of Cu from 0 to 8.6 mg 1-1. Non-mycorrhizal seedlings, which received no Cu, developed copper deficiency symptoms which were most severe at 200 mg P 1-1. Copper deficiency symptoms did not appear on the seedlings receiving no P because seedlings failed to grow. Deficiency symptoms disappeared and foliar Cu concentrations increased when seedlings were fertilized with P at 800 mg 1-1 probably because the high rates of P decreased the pH making Cu more soluble. On seedlings inoculated with G. fasciculatus and not fertilized with Cu, increasing rates of P decreased percentage mycorrhizal infection, the number of chlamydospores per g of soil, and the foliar Cu concentrations. Copper deficiency symptoms occurred only at 800 mg P 1-1 and were mild. No Cu deficiency symptoms occurred and foliar Cu concentrations were in the optimum range where Cu was applied. Apparently, P induces Cu deficiency by stimulating growth of non-mycorrhizal seedlings until Cu becomes the limiting nutrient, whereas, P-induced Cu deficiency appears to be due to P inhibition of mycorrhizal development on seedlings inoculated with G. fasciculatus.