Aluminum is rhizotoxic and is often present in acidic soils at activities high enough to inhibit root growth. The objectives of present study were to screen for Al-sensitive and Al-tolerant pineapple (Ananas comosus (L.) Merrill) cultivars and to investigate the potential mechanism(s) of Al tolerance. Seven cultivars were analysed and found to differ considerably in Al tolerance. The cultivars Soft Touch (Al-sensitive) and Cayenne (Al-tolerant) were selected for further analysis of physiological mechanism(s) of Al tolerance. The root elongation of Soft Touch was 80% compared with 120% for Cayenne in response to 300 μm AlCl₃ at pH 4.5 for 72 h. Al accumulation and Al-induced callose formation in root apices were 50 and 15% of that in Cayenne, respectively. It is clearly shown that Al only inhibited Soft Touch during the treatment, whereas it enhanced root growth of Cayenne, suggesting an Al-induced Al-tolerance mechanism operating in Cayenne. There was no significantly difference in total protein in root exudates between cultivars treated with or without 300 μm AlCl₃. However, 2D SDS–PAGE analysis could detect an acidic and low molecular weight protein in Al-treated Cayenne root tips, but not in control Cayenne or in Soft Touch both in the presence and absence of Al. The identification of organic acids in collected root exudates was conducted on Al-tolerant Cayenne. Citrate, malate, and succinate were found in Cayenne root exudates, and citrate was induced by Al exposure. Changes in organic acids from root exudates and soluble protein of root tips may be involved in the Al-tolerance mechanism. Further studies are, however, needed to clarify their functions on detoxification of Al in the pineapple roots.