The rice leaffolder (LF) Cnaphalocrocis medinalis (Guene.) folds leaves and scrapes off the green tissues. LF infestations are apparent after flowering. The folding of leaves and tissue removal result in a loss of effective leaf area for photosynthesis. The relationship between infested leaf area and photosynthetic activity is not yet fully understood (de Jong 1992, Yamamoto et al 1997). Leaf gas exchange of infested flag leaves was measured to quantify this relationship, which is important for simulating changes in assimilate production and yield.