The ability of an unidentified mycobiont from Pisonia grandis mycorrhizas to utilize organic nitrogen and phosphorus sources was determined in axenic liquid cultures. The fungus utilized inorganic phosphate and both freely- and poorly-soluble salts of inositol hexaphosphate as sole phosphorus sources. With the exception of uric acid which did not support growth, the yield of the mycobiont on all organic nitrogen sources tested was significantly greater than the yield on either NH4+ or NO3−. It is suggested that the major ecological role of the Pisonia mycobiont in stands of P. grandis on coral cays may be rapid absorption of allantoin and urea produced during degradation of uric acid by the soil microflora. This may be important in retaining nitrogen in the soil[ratio ]plant system prior to loss from the coral cay by further degradation, volatilization and/or leaching.