Bacteria of Shigella species are responsible for bacillary dysentery (shigellosis) in humans. A characteristic feature of these bacteria, used for identification purposes, is their inability to use lactose as a carbon source, in contrast to the closely related species Escherichia coli. We recently isolated clones of Shigella exhibiting a Lac+ phenotype on Drigalski medium and we are attempting to identify the gene(s) involved in this phenotype.