Root cap cells of two cotton species elicited a specific chemotactic response in zoospores of Pythium dissotocum. When roots of cotton seedlings were placed into a suspension of P. dissotocum zoospores, there was immediate attraction, accumulation and encystment exclusively in the root cap cell region. Furthermore, root cap cells remained attractive when isolated from the root: attraction, accumulation, and encystment on individual root cap cells occurred within seconds after contact. Zoospores penetrated and killed isolated root cap cells within 15-30 minutes, and seedlings died within 24 hours. In contrast, zoospores of P. catenulatum, which exhibited a chemotactic response to roots of Bentgrass, were not attracted to and did not infect seedlings or isolated root cap cells of cotton. Preliminary studies indicate that both Pythium species are capable of infecting cotton seedlings in sand culture, though it is not known if either are pathogens on cotton grown in the field.