The woody legume Prosopis glandulosa (mesquite) growing in the California Sonoran Desert develops functional root symbiotic associations (N2-fixing nodules, vesicular-arbuscular mycorrhizal fungi) at depths greater than 4 m in moist soil above a seasonally stable water table. Population densities of symbiotic microorganisms are substantially greater at depth than near the surface. Inferences of plant symbiotic dependence based upon examination of surface roots and soil may be incorrect since deep roots can support the symbioses which are critical for plants utilizing deep water.