The Escherichia coli flaA gene product (also called cheC) plays a crucial role in switching flagellar rotational direction during chemotactic responses. Wild-type and mutant alleles have been cloned onto plasmid vectors, and the gene product has been identified as a 37,000-dalton protein. The flaA product appeared as a soluble protein in the cytoplasm when overproduced in minicells and maxicells. The protein could not be detected in flagellar basal structures purified from a wild-type strain. To assess the effects of altered flaA expression, the gene was fused to a synthetic tac promoter that could be regulated by the addition of an inducer. Overproduction resulted in strong counterclockwise flagellar rotational bias and partial paralysis of flagellar motors. These results suggest that the flaA protein provides the interface between the flagellar machinery and the chemotaxis signaling system in a motor structure external to the basal body.