One hundred and two strains representing 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. The data were subjected to a number of numerical analyses. The results (confirmed by the use of overlap statistics) showed that four phenons can be distinguished within the genus shigella. These correspond with the species S. dysenteriae. S. flexneri, S. boydii and S. sonnei. 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 routine identification of strains of Shigella and other enterobacteria is discussed. Strains of the genus Providencia clustered separately from the shigellae. Three distinct species were evident within this genus: P. stuartii, O. alcalifaciens and a new Providencia species. The latter can be equated with the 'BG3' group in the DNA-DNA hybridization studies of Brenner et al. (1978).