Ectomycorrhizas have been discovered on the root system of Pisoniagrandis R. Br. (Nyctaginaceae) collected from two islands in the Capricorn group of the Great Barrier Reef, Australia. These mycorrhizas are unusual in that the Hartig net is poorly developed but instead transfer cells develop in the epidermis and cortex of the host root. The wall labyrinth is located precisely on host wall regions abutting the fungus. It is suggested that the development of wall protuberances, with an accompanying amplification of host membrane surface area exactly in the region where transfer between the two symbionts is most likely to occur, is an alternative strategy to Hartig net formation. Pisonia grandis is usually found in locations colonized by seabirds as nesting sites and therefore rich in guano. It is unusual to find an ectomycorrhiza in such a situation, where there is a high input of organic nitrogen and phosphorus at least once a year in addition to a permanently high calcium level, and the nutritional aspects of this association deserve further study.