The effects of Cr, Ni, Zn, and P exposure on the root anatomic structure, growth, and chlorophyll a concentration of Pistia stratiotes L. were studied. Plastic aquaria containing 50 g of wet plants and 5 L of pond water added with the contaminant(s) were disposed. The treatments were: (1) Cr, (2) Ni, (3) Zn, (4) P, (5) Cr + Ni + Zn, (6) Cr + Ni + Zn + P, and (7) control. Contaminant additions were done seven times. In each addition, concentrations of 1 mg of metals or 5 mg of P per liter of water were added. Chlorophyll a was an indicator more sensitive to Zn and Cr toxicity than the relative growth rate. Ni and Cr + Ni + Zn treatments were the most toxic ones, in which biomass and the root anatomical parameters (root length, cross-sectional areas [CSAs] of root, stele, and metaxylem vessels) decreased significantly. The addition of P to the treatment with combined metals attenuated the decrease in plant growth and root length, and caused a significant increase in CSAs of total metaxylem vessels, suggesting that P increased the tolerance of P. stratiotes to metals. This fact has important implications for the use of this macrophyte in constructed wetlands for industrial wastewater treatment.