The silverleaf whitefly (SLW), Bemisia argentifolii Bellows and Perring, is among the most noxious pests of numerous field and vegetable crops, causing billions of dollars worth of damage through direct feeding and massive deposition of honeydew (Byrne et al., 1990; Perring et al., 1993; Brown et al., 1995). Nevertheless, very little has been reported on the physiological mechanisms via which SLW infestation impairs plant productivity. A reduction in net photosynthetic rate (Pn) and stomatal conductance caused by SLW has been reported in tomato (Buntin et al., 1993) and cotton (Shtaynmetz, 1990; Yee et al., 1996). However, it is unclear whether the reduced stomatal conductance is a cause or consequence of the reduced Pn. Our previous work has shown that relative Pn (infested/non-infested) is correlated with SLW infestation and provided an indication that the reduced Pn is more likely due to non-stomatal limitations than stomatal factors (Lin et al., 1999). This study was aimed at characterizing the mechanisms by which SLW impairs the photosynthetic activity in cotton plants.