One hundred and two strains representing the four species of the genus Shigella and sixty-four strains representing fourteen other genera of the family Enterobacteriaceae were tested for 192 characters based on their morphology, biochemistry and physiology. These data were subjected to a number of numerical analyses. The results (confirmed by the use of overlap statistics) showed that four taxonomically-valid taxa are present within the genus Shigella. These correspond with the species S. sonnei, S. flexneri, S. boydii and S. dysenteriae. On the basis of the results, the latter three species have been redefined. Of the other bacteria studied, the Alkalescens-Dispar group was most closely related to the genus Shigella and the need for this group to be studied further is noted. The importance of using computer-based matrices for the identification of strains of Shigella and other genera is discussed. Strains of the genus Providencia clustered separately from the shigellae. Three distinct species were evident within this genus. These were P. stuartii, P. alcalifaciens and a new Providencia species. The latter can be equated with the "BG3" group in the DNA-DNA hybridisation studies of Brenner et al. (1978). The effect of R plasmids on the taxonomy of Shigella was studied by transferring R plasmids from, nine E. coli strains into seven chosen strains of Shigella. A putative R plasmid was also cured from one strain of Shigella by ethidium bromide treatment. The numerical taxonomic results for these genetically-altered strains showed that loss or acquisition of a single R plasmid did not significantly alter the classification of the strains. However, in certain instances taxonomically-important characters (such as pas production, ornithine decarboxylation and carbohydrate fermentation) were altered, which could result in mis-identification of R plasmid-carrying Shigella strains.