Aluminum (Al) phytotoxicity is the primary limitation for crop production in acid soils. Long-term sustainable agricultural production requires both the use of Al-resistant cultivars and of adapted agronomic measures. Common bean is the world's most important food legume, with an annual production value of over US$10 billion. It is produced mainly on small-scale farms (80% of dry bean production) in developing countries in Latin America and Africa. About 40% of the bean-growing area is affected by Al toxicity, resulting in decrease of grain yield from 30% to 60%. It needs significant improvement in Al resistance to reduce the dependence of small farmers on lime and fertilizer inputs. Genetic variation exists for acid soil adaptation among common bean genotypes. Field screening of 5000 germplasm accessions and bred lines in Al-toxic soils with and without lime (70% Al saturation) and participatory evaluation with farmers have resulted in identification of several promising genotypes. Field screening for Al resistance would seem to be the most desirable approach, because it best approximates the intended cropping environment. In practice, however, reliable ranking of genotypes in the field can be difficult. This is mainly because exchangeable Al levels may not be uniform and also environmental factors could interact with soil Al to mask the expression of Al resistance. Thus it is necessary to combine field screening with greenhouse screening techniques based on physiological traits of Al resistance. The most frequently measured effect of excess Al is inhibition of root elongation. Nutrient solution studies are better suited for determining more precisely the Al activity that is influencing root elongation, exclusive to other associated acid-soil-stress constraints. In order to identify genotypes resistant to Al, additional ions in solution that may be toxic, such as H+ concentration, and ions that may help alleviate Al toxicity, such as Ca2+, must be controlled. Nutrient solution culture allows evaluation of a large number of genotypes quickly and could be very useful for identification of (i) parental genotypes with contrasting root architecture for bean breeding, (ii) contrasting genotypes for physiological analysis, (iii) QTLs related to Al resistance, and (iv) candidate genes associated with Al resistance in common bean. We developed a hydroponic screening methodology using low ionic strength nutrient solutions to evaluate root traits of seedlings grown under greenhouse conditions. Seeds were germinated using a filter paper for 2 to 3 days and seedlings with uniform root length (5-7 cm) were selected for evaluation with nutrient solution, composed of (in µM) 286 CaSO4.2H2O, 300 KNO3, 150 NH4NO3, 2.5 NaH2PO4.H2O, 150 MgCl2.6H2O, 14 CaCl2.H2O, 5 FeCl3.6H2O, 5 Na2EDTA.2H2O, 1 MnCl2.4H2O, 1 ZnCl2, 0.2 CuCl2.2H2O, 6 H3BO3, 5 Na2SiO3.9H2O, 0.001 NaMoO4.2H2O and 57.5 NaCl. We used 2 treatments (pH 4.5 without Al and pH 4.5 with Al of 50 µM). Changes in root elongation as influenced by Al treatment were monitored by measuring length of the primary root at 0 and 3 days after Al treatment. Seedlings were harvested and root systems were scanned using a software program, WINRHIZO and a flatbed scanner. Root attributes including total root length, average root diameter, number of root tips and root volume were recorded. Shoot and root biomass were also recorded after drying the tissue in an oven at 65 °C for 2 days. Using this hydroponic screening method, we evaluated the rate of elongation of the primary root and the root morphology of 53 bean genotypes (landraces and breeding lines) and 66 RILs (Recombinant Inbred Lines) of G 5273 x MAM 38 to identify Al resistant genotypes. We identified 3 Andean genotypes (G19833, BRB 191 and G 5273), 8 Mesoamerican genotypes (G 1261, MAR 1, DOR 714, FEB 190, G 11015, G 3513, A 774 and G 855) and 4 RILs of G 5273 x MAM 38 (HF14137-19, HF14137-26, HF14137-98 and HF14137-88) with greater level of Al resistance. We also found that four root traits, percent inhibition of root elongation, percent increase of average root diameter, total root length per plant and total number of root tips per plant could serve as screening tools to identify Al resistant common bean genotypes. Screening of 30 germplasm accessions from a sister species, Phaseolus coccineus, resulted in identification of 3 accessions with high level of Al resistance. These accessions could serve as parents to incorporate high level of Al resistance in common bean through genetic enhancement. Work is in progress to further characterize physiological mechanisms of Al resistance, a prerequisite for development of simplified screening procedures and identification of quantitative trait loci (QTLs) for Al resistance in common bean. We are following a scheme that combines physiological analysis, QTL mapping for specific traits, and field phenotypic expression of acid soil tolerance, to associate traits with tolerance and to determine the relative importance of different traits.