In a study of dominant diazotrophic bacteria present in the rhizosphere of wheat and maize, 28 strains of Enterobacteriaceae were isolated. They were all Voges-Proskauer positive, motile at 28 °C but not at 37 °C, and they produced a Tween-80 esterase and did not exhibit decarboxylase activity. This fits well with the description of Rahnella aquatilis. The ability of these strains to reduce acetylene in pure culture and in association with their host plant and the DNA hybridization with a nifHDK probe are described. This is the first time that R. aquatilis is reported as a rhizosphere-associated bacterium and also a nitrogen fixer. Key words: Rahnella aquatilis, rhizosphere, wheat, maize, nitrogen fixation.