Movement on surfaces, or swarming motility, is effectively mediated by the lateral flagellar (laf) system in Vibrio parahaemolyticus. Expression of laf is induced by conditions inhibiting rotation of the polar flagellum, which is used for swimming in liquid. However, not all V. parahaemolyticus isolates swarm proficiently. The organism undergoes phase variation between opaque (OP) and translucent (TR) cell types. The OP cell produces copious capsular polysaccharide and swarms poorly, whereas the TR type produces minimal capsule and swarms readily. OP7TR switching is often the result of genetic alterations in the opaR locus. Previously, OpaR, a Vibrio harveyi LuxR homolog, was shown to activate expression of the cpsA locus, encoding capsular polysaccharide biosynthetic genes. Here, we show that OpaR also regulates swarming by repressing laf gene expression. However, in the absence of OpaR, the swarming phenotype remains tightly surface regulated. To further investigate the genetic controls governing swarming, transposon mutagenesis of a TR (⌬opaR1) strain was performed, and SwrT, a TetR-type regulator, was identified. Loss of swrT, a homolog of V. harveyi luxT, created a profound defect in swarming. This defect could be rescued upon isolation of suppressor mutations that restored swarming. One class of suppressors mapped in swrZ, encoding a GntR-type transcriptional regulator. Overexpression of swrZ repressed laf expression. Using reporter fusions and quantitative reverse transcription-PCR, SwrT was demonstrated to repress swrZ transcription. Thus, we have identified the regulatory link that inhibits swarming of OP strains and have begun to elucidate a regulatory circuit that modulates swarming in TR strains.Vibrio parahaemolyticus is a gram-negative marine bacterium found in coastal and estuarine waters worldwide (11, 31). It possesses two distinct flagellar systems: the polar system produces a single sheathed flagellum designed for swimming motility, whereas the lateral flagellar (laf) system is adapted for movement over moist or viscous surfaces (3, 46). The lateral flagella are expressed only when the cell senses a surface environment (45). Swarmer cells are elongated (5 to 20 times the length of the swimmer cell) and multinucleoid, and they produce a multitude of lateral flagella. Two environmental signals are known to be required for laf expression: iron-limiting conditions and inhibition of polar flagellar rotation (32,43,44). The polar flagellum acts as a tactile sensor for the cell. When the cell encounters a surface or sufficiently viscous environment, flagellar rotation is impaired and laf is induced. Thus, the lateral flagellar system is intimately linked to the polar system. However, the molecular mechanism for sensing polar flagellar inhibition and the signal transduction pathway regulating laf expression are not known. It was the goal of this work to begin to elucidate the regulatory circuit controlling laf expression.Many bacterial species are known to swarm, including some with mixed polar and pe...