Bacterial chemotaxis presents a model sensory system in which cells modulate the direction of rotation of their flagella in response to gradients of certain chemicals. The chemotactic machinery ofEscherichia coli is currently being systematically reduced to its individual components through the accomplishments of behavioral, physical, genetic, molecular genetic, and biochemical analyses. Thirteen of the so called "MCP-related" class of chemotaxis gene products are known. Transmembrane methyl-accepting chemotaxis proteins (MCPs) are important for the chemical sensing, signal generation, and sensory adaptation processes. Soluble chemotaxis proteins relay information from these MCPs to structural components of the flagella referred to as switch proteins. Emphasis here is on the separate roles each of these groups of chemotaxis proteins perform, as well as their protein-protein relationships.