Abstract.1. The effects of leaf toughness on mandibular wear of the leaf beetle Plagiodera versicolora Laich. (Coleoptera: Chrysomelidae) were evaluated by feeding adult beetles young (tender) and old (tough) leaves of Salix babylonica and S. alba ‘Tristis’.2. Tough leaves erode the cutting surface of beetle jaws more so than tender leaves.3. Beetles with worn jaws consumed leaves at a slower rate than ones with less mandibular wear.4. Because rates of leaf consumption and egg production are positively correlated, increased mandibular wear may reduce beetle fecundity.5. These results support the belief that leaf toughness may act as a potent defence affecting morphology, feeding behaviour, and ultimately spatial and temporal patterns of herbivo