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Nitrogen-15/14 isotope ratios (sigma N-15) were measured in groundwater, infiltrating wastewater, and natural uncontaminated soil to determine the source of high NO3- concentrations in two desert basins at Fort Irwin National Training Center in the Mojave Desert. The abundance and isotopic composition of dissolved nitrogen gas were measured in some of the groundwater samples to determine the effects of denitrification on the abundance and isotopic composition of the NO3-. Delta N-15-NO3 values in groundwaters most likely to include sewage effluent ranged from about II to 23 parts per thousand. Delta N-15 values in groundwater not near sources of sewage effluent ranged from 6 to 11 parts per thousand, a range similar to that found in the overlying soils, which contain large concentrations of leachable NO3- presumed to be of natural origin. Some of the samples with relatively low NO3- concentrations and high delta N-15 values contained excess non-atmospheric nitrogen gas attributed to denitrification. These data indicate that high NO3- concentrations in groundwater unaffected by sewage effluent may result from leaching of naturally occurring nitrogen in desert soils, and that some of the NO3- may be removed by denitrification at the water table.