Flagellin genes (fliC) were detected in two species of the genus Shigella. The fliCSF gene cloned from Shigella flexneri produced normal-type flagella in an Escherichia coli delta fliC strain while the fliCSS genes from two Shigella sonnei strains produced curly-type flagella and their expression is repressible by Salmonella FljA repressor. The fliCSF gene (1650 bp) shared high similarity with the E. coli fliCE gene not only in the 5' and 3' constant sequences but also in the upstream and downstream sequences. The fliCSS genes (1572 bp) shared high similarity with the Salmonella typhimurium fliCS gene in the operator and 3' constant sequences and also shared high similarity with the fliCE gene in the downstream sequence, suggesting that the fliCSS gene has undergone horizontal transfer and recombination. Differences in nucleotide sequences of the central variable regions among the four fliC genes, including fliCE and fliCS, suggest that they started differentiation in each lineage approximately 80 million years ago. Loss of motility in Shigella seems to be evolutionarily a recent event.