Swarming is an adaptation of many bacteria to growth on surfaces. A search for genes controlling swarmer cell differentiation of Vibrio parahaemolyticus identified a novel three-gene operon that potentially encodes a pyridoxal-phosphate-dependent enzyme, an extracellular solute-binding protein, and a membrane-bound GGDEF- and EAL-motif sensory protein. The functions of these motifs, which are named after conserved amino acid sequences, are unknown, although the domains are found singly and in combination in a variety of bacterial signaling proteins. Studies with translational fusions supported the predicted localization of the gene products. When the operon was overexpressed, swarmer cell gene transcription was induced in liquid culture. Mutants with defects in any of the three genes exhibited decreased swarming and lateral flagellar (laf) gene expression. Complementation studies confirmed an operon organization and suggested that all three genes participated in laf regulation. The lesions that decreased swarming increased capsular polysaccharide (CPS) production, and overexpression of the operon inhibited transcription of the CPS gene cpsA. Thus, the scrABC locus appears to inversely regulate two gene systems that are pertinent to colonization of surface swarming and CPS.