Difficulty has been experienced in establishing a suitable aluminum (Al) index to predict Al toxicity to plants grown in nutrient solutions with a wide range of properties. In the present study, relationships were evaluated between root length and (i) concentration of total Al, (ii) concentration of monomeric Al, and (iii) the sum of the activities of monomeric Al species (SaAlmono) in solution. Results are reported for soybean (Glycine max(L.) Merr.), subterranean clover (Trifolium subterraneumL.), alfalfa (Medicago sativaL.), and sunflower (Helianthus annuusL.). Total Al concentration in solution, comprising polymeric and monomeric Al species, was a poor index of Al toxicity, confirming the hypothesis that only monomeric Al is toxic to root growth. In solutions with widely differing composition, the concentration of monomeric Al also proved unsatisfactory due to ionic strength effects on the activities of monomeric Al species. SaAl monowas the best index of Al toxicity, accounting for 72 to 92 % of the variation in root length depending on the plant species. Root length was reduced by 50% at SaAl monoof 7–16 μM in soybean, 13 μM in subterranean clover and alfalfa, and 11 μM in sunflower.