The mechanism responsible for inhibition of the establishment of mycorrhizal associations in Sorghum vulgare Pers. (herbaceous monocot) and Citrus aurantium L. (woody dicot) under high levels of soil phosphorus (P) was studied. Plants were grown on low fertility loamy sand (4.5 ppm P), receiving superphosphate [Ca (H2PO4)2H2O] at 0, 6, 28, 56, 228 and 556 ppm P along with all the other necessary nutrients. The percentage P content of root tissue was correlated with the amount of P added to the soil. Root exudation, measured in terms of the net leakage of soluble amino acids and reducing sugars from the roots within a 17–h period, was significantly higher under low P levels (0, 6 and 28 ppm P) than under high P levels (56, 228 and 556 ppm P). The amount of exudation was correlated with a P-induced decrease in phospholipid levels and associated changes in permeability properties of root membranes, rather than with changes in the root content of sugars and amino nitrogen. The hypothesis is proposed that phosphorus inhibition of mycorrhizal symbiosis is associated with a membrane-mediated decrease in root exudation.