The generally lower 15N/14N ratio of nitrogen-fixing organisms provides an opportunity to screen mixed populations for individuals fixing nitrogen. Samplings of associated California native species, along with the soil on which they were found, show differences in 15N/14N ratios suggesting a capability among some for the fixation of nitrogen. Not all suspected fixers were depleted in 15N, and two saprophytes had significant 15N enrichment. The method shows some promise, but further understanding of fractionation processes is needed before the method can be generally applied.