The results of continued biochemical and serological studies on a number of previously described provisional Shigella serotypes are reported. On the basis of the results obtained it is suggested that provisional serotypes 58 and 2050-52 may now be added to Shigella subgroup A as Shigella dysenteriae 9 and S. dysenteriae 10, respectively, and that serotypes 123, 425, 2770-51, and 703 may be added to Shigella subgroup C as Shigella boydii 12, S. boydii 13, S. boydii 14, and S. boydii 15, respectively. It is suggested that the status of serotypes 3873-50, 3615-53, and 1621-54 remain provisional for the present. Also, the biochemical and serological relationships of three new provisional Shigella serotypes are reported. These are serotypes 2000-53, 3341-55, and 2710-54.The status of several other serotypes, some of which were previously described as provisional shigellae, have been investigated and discussed and it is concluded that these are not shigellae but anaerogenic Escherichia coli instead.