The influence of a vesicular-arbuscular (VA) mycorrhizal fungus, phosphorus (P) fertilization, and soil microorganisms on growth and root architecture of big bluestem (Andropogon gerardii Vitroari) was investigated. In pasteurized soil, mycorrhizal inoculation significantly improved plant growth and increased root length and the number and the diameter of the primary, secondary and tertiary roots. These differences between mycorrhizal and non-mycorrhizal plants diminished with added P. In pasteurized soil amended with non-sterile soil sievate, differences between mycorrhizal and non-mycorrhizal plants were still obvious, but in many treatments these plants grew more poorly (had less dry weight, root length, number or diameter) than their counterparts in unamended pasteurized soil. Growth in non-sterile soil was also suppressed, and mycorrhizal responses were not detected since all of the plants in non-sterile soil became mycorrhizal whether or not they were inoculated. Two analyses of calculated parameters which describe root-system architecture were conducted. The first, specific root length (SRL), revealed that mycorrhizal symbiosis dramatically alters root morphology in soils of low fertility. These changes were similar to the changes evoked by added P. The second, path length (Pe) revealed that mycorrhizal fungi (and to some degree other soil microbes) significantly alter root architecture by reducing the relative amount of root branching. Apparently, mycorrhizal plants develop a more elongate, exploratory growth pattern which permits the fungal hyphae to extract nutrients from a larger volume of soil. In contrast roots of non-mycorrhizal plants maintain a more highly branched pattern of root growth, and the roots themselves play a more critical role in the direct extraction of nutrients from the soil. These differences in root topology were not directly associated with the concentration of exogenous P, but instead appeared to be controlled by the mycorrhizal fungi themselves. Thus, while internal P content of plants mediates the establishment of the mycorrhizal symbiosis, the fungi can alter the root architecture of the plant to a form which best accommodates the symbiosis under the prevailing fertility and rhizosphere conditions in the soil. By altering root-system architecture in this manner, the mycorrhizal fungi can control, at least to some degree, the dependence of the host on the symbiosis. Thus, the topology of the root system is contingent upon the microflora in the rhizosphere. The topological analysis revealed differences in root architecture not detected by any of the other measures of root morphology. These differences suggest that mycorrhizal fungi affect root architecture and plant growth in ways not directly associated with phosphorus uptake.