A chemostat was used as a model system to study competitive interactions of diazotrophic microorganisms. Enrichment experiments were carried out under microaerobic conditions (8.7 μmol O2/l) with malate as the sole carbon source. The starting material was a Korean rice soil including intact root pieces. The enrichment process was governed by the dilution rate. High dilution rates resulted in the enrichment ofAzospirillum lipoferum, whereas low dilution rates led to the predominance of an unidentified organism, named Isolate R. Dilution rates were set in the range from D=0.005 to D=0.1 h−1. The growth kinetics of both organisms followed Monod's model in the enrichment culture. From the experiments, the maximum specific growth rate ofA. lipoferum and Isolate R were 0.069 h−1 and 0.025 h−1, respectively. The corresponding Ks-values were 8.4 and 0.9 (mg. 1−1). The point of theoretical coexistence of both organisms was calculated to occur at a substrate concentration of s=3.0 (mg.l−1) with a growth of rate μ=0.018 h−1. Hence the preset nutritional niches occupied by at least two organisms.Azospirillum lipoferum seems to represent the copiotroph microflora and Isolate R is of the oligotroph type. In addition to its high substrate affinity Isolate R liberatedca. 75% of the fixed nitrogen into the medium, which indicates its potential role for mutualistic interactions in the rhizosphere.