A DNA-DNA hybridization study was carried out to determine the taxonomic position of a new group of enterobacteria (group H2) previously studied by numerical taxonomy. All the strains of this group revealed relatively high reassociation binding ratios with the centrotype; 82% of the strains of the group showed more than 69% of reassociation with the centrotype. In spite of numerical taxonomy conclusions, there was no genetic relationship with the species Enterobacter cloacae (higher reassociation binding ratio: 37%). No significant genetic relationship with the other groups of enterobacteria was found. Rahnella aquatilis was defined from phenotypic and genetic data. The strain 133 (CIP 78-65) is proposed as type strain of the species.