Spatial variations in nitrogen availability were studied in a desert community codominated by Larrea tridentata (DC.) Cov. and Prosopis glandulosa Torr. Measurements of natural 15N values in tissues suggested that Prosopis obtains approximately half of its nitrogen through direct symbiotic fixation. Soils were collected under 1) Prosopis shrubs, 2) Larrea shrubs < 2 m from Prosopis (LP), and 3) Larrea< 2 m from other Larrea but > 5 m from the nearest Prosopis (LL). Prosopis soils showed significantly higher rates of nitrogen mineralization than LL soils in both A and B horizons. Rates of mineralization in LP soils were significantly higher than rates in LL soils only in the B horizon and were not significantly different from rates in Prosopis soils. Leaf nitrogen concentrations were significantly higher in LP shrubs (2.06%) than in LL shrubs (1.78%), although 15N values did not differ between the two shrub types. Nitrogen concentrations in Perezia nana Gray, a perennial herb, were greater in plants under Prosopis shrubs (2.09%) than under LP shrubs (1.93%) or LL shrubs (1.67%). Despite apparent differences in nitrogen availability, biomass of Larrea and density of Perezia did not differ significantly among these sites.