In a study of adaptive traits of rhizosphere inhabiting N2-fixing bacteria, 56 strains representative for the diazotrophic population were isolated from the rhizosphere of three week old spring wheat plants (cv. Castan). For the isolation procedure, a medium was used which contained the exudates of sterile plantlets of wheat (the ''spermosphere model''). N2-fixing Bacillus polymyxa and Bacillus circulans were dominant in three out of four French soils studied. The dominant N2-fixing microflora of the fourth soil was a population of Rahnella aquatilis, an enteric bacterium closely related to Enterobacter agglomerans and Erwinia herbicola. The sizes of these N2-fixing populations were between 1 and 5 x 10(5) cfu g-1 dry weight of rhizosphere soil. These two rhizosphere populations differed markedly by two important characters: (1) when associated with plants under gnotobiotic conditions, the average acetylene reduction activity was much higher in Bacillus than in R. aquatilis strains; (2) in vitro, 90% of Bacillus isolates exhibited a clear antagonistic activity against Gaeumannomyces graminis var. tritici (Ggt), the causative agent of the take-all disease of wheat, whereas none of the 25 strains of R. aquatilis tested showed any antogonistic activity.