In an experiment to determine the influence of mycorrhizal colonization on root development, fertilized seedlings of cotton were grown in long tubular pots. Inoculation of soil with an isolate of Glomus mosseae at 10, 25 or 40 cm below the soil surface resulted in spread of arbuscular mycorrhizas up and down the root system, and localized elongation of roots following colonization. Specific root length was not influenced by colonization. Increases in local root density at the point of inoculation were observed, though total root mass in relation to shoot mass declined following initiation of mycorrhizas.