Virtually all organisms have means of monitoring their environment and making use of information gained to aid their survival. Many organisms, from bacteria to animals, move from place to place and can alter their movements. Chemotaxis is a signal transduction system found in motile bacteria that allows them to sense changes in the concentrations of various extracellular compounds and change their swimming behavior in a way that moves them toward more favorable environments. Chemotaxis is the most ancient sensory-motor process in nature. For years, studies of enteric bacteria, such as Escherichia coli and Salmonella typhimurium, have served as the paradigm for understanding this process on a molecular level. Recent studies on the gram-positive bacterium, Bacillus subtilis, and other bacteria, suggest that a slightly more complex system may be ancestral to that of the more extensively studied enterics. Aspects of chemotaxis that are unique to B. subtilis include a more complex adaptation system, with protein-protein methyl group transfer, chemotaxis proteins having no counterparts in E. coli, and a very extensive repertoire of repellents that are sensed at very low concentrations by receptors.