Root architectural plasticity might be an important factor in the acquisition by plants of immobile nutrients such as phosphorus (P). In this study, we examined the effect of P availability on the orientation of basal roots with respect to gravity, and thereby on the growth angle of these roots of common bean (Phaseolus vulgaris L.). In one set of studies the growth angle of basal roots of bean seedlings was measured over time. Sixteen bean genotypes were examined; six showed a decrease in root orientation with respect to gravity in low P media, one increased orientation, and nine showed no difference within 5 d of basal root emergence. Bean taproots also showed decreased root orientation with respect to gravity in low P. Growth angle after 5 d was correlated with field performance of contrasting genotypes in low P tropical soils. Mineral deficiencies other than P did not cause changes in root angle. In a split pouch system that provided high or low P solution to different parts of the root system, the decrease in root angle in low P was found to be a response to global P availability, and not local to the portion of the root system in low P. Effects of P availability on root angle were associated with reduced shoot P concentration, but preceded effects on plant biomass accumulation and leaf area expansion. Results from growth pouches for genotype G 19833 were confirmed using a solid-phase buffered sand-culture system supplying P at three levels. Pea (Pisum sativum), soybean (Glycine max Williams), chickpea (Cicer arietinum), lima bean (Phaseolus lunatus), and lentil (Lens culinaris) were grown with and without P; soybean and pea also showed decreased basal root angles in low P.