Motile bacteria are attracted to a variety of chemicals — a phenomenon called chemotaxis (for a review, see Weibull, 1960). Although chemotaxis by bacteria has been recognized since the end of the nineteenth century, thanks to the pioneering work of Engelmann, Pfeffer, and other biologists, the mechanisms involved are still almost entirely unknown. How do bacteria detect the attractants? How is this sensed information translated into action; that is, how are the flagella directed?