The ability of the external mycelium in arbuscular mycorrhiza for N uptake and transport was studied. The contribution of the fungal symbiont to N acquisition by plants was studied mainly under waterstressed conditions using 15N. Lettuce (Lactuca sativa L) was the host for two isolates of the arbuscular mycorrhizal fungi Glomus mosseae and G. fasciculatum. The experimental pots had two soil compartments separated by a fine mesh screen (60 μm). The root system was restricted to one of these compartments, while the fungal mycelium was able to cross the screen and colonize the soil in the hyphal compartment. A trace amount of 15NH + 4 was applied to the hyphal compartment 1 week before harvest. Under water-stressed conditions both endophytes increased the 15N enrichment of plant tissues; this was negligible in nonmycorrhizal control plants. This indicates a direct effect of arbuscular mycorrhizal fungi on N acquisition in relatively dry soils. G. mosseae had more effect on N uptake and G. fasciculatum on P uptake under the water-limited conditions tested, but both fungi improved plant biomass production relative to nonmycorrhizal plants to a similar extent. The work was included in the CICYT Environmental Projects. Peer reviewed