Aluminum speciation is recognized to associate with environmental pH. In this paper, the effects of aluminum speciation (A1a, A1b) on the growth, protein content, acid phosphatase (AP) activity of wheat root were studied by changing the pH of aluminum solutions. Two concentrations of total aluminum, 25 micromol x L(-1) (T1) and 75 micromol x L(-1) (T2), were set up in the test solutions, and four levels of pH, 4.0, 4.5, 5.0 and 5.5, were regulated for each solution. The results indicated that the growth of wheat root in T1 and T2 was strongly inhibited by low doses of Ala and Alb, while the changing trend of protein content and AP activity varied when exposed to different composition of active A1. When the A1a concentration was above 90% of the active Al concentration, the protein content was reduced while the AP activity was enhanced by Al at pH 4.0, but it was reverse at pH 5.0 when the concentration of Ala and Alb was below 10 micromol x L(-1), respectively.