The flaA locus of Vibrio parahaemolyticus encodes one of the four polar flagellin genes, the flagellum-capping protein HAP2, and three additional flagellar genes. Sequence analysis downstream of the gene encoding HAP2 revealed the region to be similar to the fliD (HAP2) locus of Pseudomonas aeruginosa. The deduced protein sequences for the newly identified genes suggest that one protein belongs to the family of transcriptional regulatory proteins known to interact with σ54, one may be a rod component of the flagellum, and one resembles the FMS protein. fliS is an essential flagellar gene in many bacteria; however its function is not clear. The V. parahaemolyticus polar flaC flagellin gene was poorly expressed in Escherichia coli. Production of FlaC was stimulated by provision of the flaA locus in trans. Dissection of this locus revealed that the fliS-like gene, flaJ, was required for increased expression of flaC. Stimulation by FlaJ occurred in E. coli mutants defective in either the master flagellar-controlling operon or the gene encoding the flagellar σ28. Therefore the effect of FlaJ was not mediated through flagellar proteins. Nor was it mediated through σ54, for enhanced FlaC production was observed in mutants with defects in the gene encoding σ54.