Chemotaxis of cholera vibrios facilitates the association of these bacteria with the mucosal surface. 2) Mucosal extracts can block the chemotactic receptors on the bacterial surface and thereby retard the association of chemotactic bacteria with the mucosal surface. 3) Chemotactic cholera vibrios grow more efficiently than non-chemotactic mutants in germfree mice and in isolated intestinal loops of rabbits. Conversely, non-chemotactic mutants grow more efficiently in infant mice. 4) Chemotaxis is therefore of considerable importance to the in vivo growth of cholera vibrios. The various mechanisms, including those of local immunity, by which bacterial chemotaxis can be exploited for the benefit of the host deserve further exploration.