Twenty winter cultivars of Triticum aestivom L. (wheat) were grown in solution culture with and without aluminum (A1) (74 gM, 2.0 mg L-') for 14 days. Exposure to s increased root growth of the most tolerant cultivar, while both root and shoot growth were depressed in all other cultivars. On the basis of a root tolerance index (RTI = weight of roots grown with A1/ weight of roots grown without A1), cultivar tolerance to A1 ranged 9-fold, from 0.13 + 0.01 to 1.16 + 0.10. Symptoms of A1 toxicity were most evident on roots. Aluminum-affectedr oots were relatively short and thick and had numerous undeveloped laterals. Leaves of some cultivars showed chlorosis resembling iron deficiency, and others showed purple stems typical of phosphate deficiency. Plants of all cultivars grown with and without A1 depressed the pH of nutrient solutions, presumably until NH4+ was depleted, at which point the pH increased. Cultivar tolerance7 expressed both as the root tolerance index and a shoot tolerance index, was negatively correlated with the negative log of the mean hydrogen ion (H+) concentration, the minimum pH and the slope of the pH decline, each calculated from pH data collected during the first 9 days of the experimental period before any sharp rises in pH occurred. These results are consistent with the hypothesis that the A1 tolerance of a given cultivar is a function of its ability to resist acidification of the nutrient solution and hence to limit the solubility and toxicity of A1.