Shigella sonnei has become the most frequently reported cause of shigellosis in the United States. Since Shigella subgroup D has no other serotypes, colicin production has been used as a basis for differentiating and identifying epidemiologically related strains. The results of colicin typing 115 cultures of S. sonnei from eight outbreaks of shigellosis occurring in widely separated regions of the United States support the usefulness of this technique. In each outbreak, the cultures were either of the same colicin type or were uniformly untypable. Unrelated cases yielded a variety of types. Definitions of the relative frequencies and geographic distributions of the various strains of S. sonnei in the United States await an accumulation of experience with the method.