Arbuscular mycorrhizal fungi can enhance nutrient acquisition by a plant via their extraradical hyphae. This is particularly true for phosphorus, but the case for nitrogen (N) has been less clear. In our growth systems there was a small air-gap between root and hyphal compartments, which eliminated diffusion of nutrients between compartments. Moreover, our methods allowed us to distinguish between nitrate and ammonium. We found that N transfer to Zea maize L. depends on the sources fed to the hyphae of Glomus aggregatum Schenck & Smith. In experiment 1, despite the fact that plant demand for N was already met, plants received 10 times as much 15N from ammonium than from nitrate. In experiment 2, 74% of shoot-N was derived from the slow-release urea added to the hyphal compartment while only 2.9% was derived from the nitrate-N. Intraradical hyphae isolated from roots contained a considerable amount of 15N in the cell wall even when 15N-nitrate was the source. We conclude that the mycorrhizal fungus can rapidly deliver ammonium-N to the plants, and that while the fungus can absorb nitrate, it apparently lacks the capacity to transfer it to the plant.