A comparison of physiological parameters was carried out between healthy and Ganoderma infected 17-year old oil palms grown under the same field conditions. Results from gas exchange measurements indicate that stomatal conductance was significantly reduced in infected palms. This led to significant reductions in transpiration rate and intercellular CO2 concentration in the infected palms. The relative leaf chlorophyll content and quantum efficiency of PS II were also significantly reduced in the infected palms. The results indicate that infected palms were under water stress that was induced by injury to their root and vascular transport system and not related to soil water deficits.