Aluminium toxicity was investigated by analyzing the reaction of hydroponically-grown plants of two ecotypes ofT. triandrato Al (0, 25, 50, 100 μM; pH 4.1; for 28 days). The Al-tolerant 'Berg ecotype responded to the Al treatments by increasing root extension growth, D.M. production and they also controlled water-use over the full range of Al stresses. In contrast, Al inhibited root growth and D.M. production in the Al-sensitive Zululand ecotype and they also failed to control water-use at 100 μM Al. Microscopical investigation showed that the root cells of both ecotypes were generally resistant to Al with cellular disorganization being confined to the Zululand plants at the highest Al stress. The root caps of both ecotypes responded to Al in ways (amyloplast numbers, cap volume, secretions) which coincided with the contrasting growth responses observed in 'Berg and Zululand plants. Aluminium tolerance involved the accumulation of Al in the roots of 'Berg plants and was coincident with increases in root P and K concentrations. Consistent differences in water-use existed between ecotypes and control over water-use was linked to root growth. These findings are used to develop a theory covering the regulatory mechanisms involved in controlling root extension growth, Al tolerance and water-use.