SUMMARY: The taxonomic value of the different 'groups' into which the Enterobacteriaceae are divided is assessed, and consideration is given to the classification of different groups within the family. No scheme fits the requirements of all workers and two different approaches are made, one for workers in applied bacteriology, and a more logical one for taxonomic purists. For the former scheme it is suggested that common (vulgar) names should be used to describe the groups and their subdivisions. Linnaean binomials are essential for the more systematic scheme, in which the Arizona group is merged with Salmonella; BallerupBethesda is combined with Citrobacter freundii ; Sonne's bacillus and Bacillus alca~scens are included in Escherichia ; Cloaca and Hafnia (32011) are included as species of Klebsiellu, and Providence (29911) as a species of Proteus. Few of these suggestions are new, but the scheme is a classification of a family and not a collection of ' p u p s ' .In taxonomic studies it is convenient to divide organisms into groups and subgroups, leaving the status of rank for each category to be decided when order appears and the work is nearing completion. Problems of rank are met with in all types of organism; they are attacked in different ways in different disciplines, but even in one branch of microbiology there is no uniformity. Among bacteria certain organisms from the intestines of animals present special problems ; their morphological and cultural characters are alike, and resemble those of bacteria found in soil, in water and on plants. Though there is nothing to be said in favour of an ecological grouping of bacteria, in practice ecology may determine the primary grouping: a plant pathologist studies those bacteria found in plants, the water bacteriologist studies essentially similar organisms in water, and the clinical pathologist finds them in human or animal excretions. Often these ecologists work in isolation and develop independent taxonomic schemes.White (1937), writing on the construction of a taxonomic system for bacteria, contrasted determinative keys with systems based on mutual relations and differences. He said that such a system must develop from multiple foci of intensive study, which would gradually widen until they became confluent ; only then could an opinion be formed as to the rational groupings of larger size. Of necessity such a system would be built slowly, but it would be laid an sure foundations. White himself laid many of these foundations and, with Kauffmann, Jordan, St John-Brooks, Schutze and Scott of the original Salmonella Subcommittee, paved the way by making an intensive study of one group of organisms. Since then the number of groups studied has increased, and the Salmonella Subcommittee of the International Committee on Bacteriological Nomenclature has been replaced by an Enterobacteriaceae Subcommittee (E.S.) which covers many more of the 'multiple foci'. Enlarging