This work studies the classification, by numerical procedure, of 111 strains belonging to the family Enterobacteriaceae. Their taxonomic position is discussed, in relation to the genus Citrobacter, Levinea, Enterobacter, to which some appear to be closely associated. The study is based on the usual biochemical and nutritional characters as shown by the utilisation of substrates tested as sole sources of carbon and energy. Six hierarchical classes can thus be defined. Their degree of affinity with members of the Enterobacteriaceae groups or genera is discussed. The diverse origins of the strains examined (human, soil, aquatic) are certainly the cause for the individualization of the new classes.