The formation of ectomycorrhizas, a tight association between fine roots of trees and certain soil fungi, improves plant nutrition in a nutrient-limited environment and may increase plant survival under water stress conditions. To investigate the impact of mycorrhiza formation on plant water uptake, seven genes coding for putative water channel proteins (aquaporins) were isolated from a poplar ectomycorrhizal cDNA library. Four out of the seven genes were preferentially expressed in roots. Mycorrhiza formation resulted in an increased transcript level for three of these genes, two of which are the most prominently expressed aquaporins in roots. When expressed in Xenopus laevis oocytes, the corresponding proteins of both genes were able to transport water. Together, these data indicate, that the water transport capacity of the plasma membrane of root cells is strongly increased in mycorrhized plants. Measurements of the hydraulic conductance of intact root systems revealed an increased water transport capacity of mycorrhized poplar roots. These data, however, also indicate that changes in the properties of the plasma membrane as well as those of the apoplast are responsible for the increased root hydraulic conductance in ectomycorrhizal symbiosis.