Previous studies on the effects of leaf galls on leaf physiology have reported positive and negative effects. Thus, the effects of insect galls on leaf photosynthesis were investigated in hackberry (Celtis occidentalis), a tree found throughout central U.S.A. that is particularly susceptible to infestation by several gall-inducing arthropod species. These galls have different morphologies and locations on the leaves of hackberry. Effects of four types of galls on leaf function were examined. Only two types of galls, petiole galls and leaf-lamina spine galls, resulted in decreased leaf photosynthesis. These decreases were most likely the result of disturbance to the photosynthetic apparatus, not declines in stomatal conductance. Evidence is presented that petiole galls may disrupt the water supply to the leaf lamina. The results of this investigation indicate that different types and locations of arthropod galls result in no or negative effects on the physiology of infested leaves of this highly gall-bearing tree in the central U.S.A.