Summary 1. We investigated whether increases in cell wall peroxidases link the benefits of induced resistance and the potential ecological costs of reduced leaf expansion. 2. Rumex obtusifolius is a host plant of the chrysomelid beetle Gastrophysa viridula. The effect of feeding by G. viridula on plant physiology and activity of cell wall-bound peroxidase activity in the plant was measured under controlled conditions. 3. Gastrophysa viridula were applied to fully expanded leaf 4, and expansion of leaf 8 was measured over 22 days. Removal of leaf tissue by the herbivore delayed subsequent leaf emergence in treated plants and temporarily reduced the expansion rate of leaf 8, beginning 5 days after treatment. This reduction continued for 4 days when compared with non-treated plants, and reduced final leaf area by approximately 37%. 4. Final epidermal cell areas in leaf 8 were reduced by approximately 13%, while epidermal cell numbers remained unchanged. 5. Cell wall-bound peroxidase activity was measured in leaf 8 over a 7-day period following attack. Activity increased within 24 h following the start of treatment and peaked at day 3. 6. We postulate a herbivore-induced systemic mechanism linking reductions in cell expansion and subsequent leaf growth rates as a result of an increase in cell wall-bound peroxidase activity. 7. Herbivore grazing commonly reduces subsequent host growth, and the assumption has been that this is largely due to the consequences of leaf area lost to grazing. This study suggests that reduced growth, and the associated ecological costs result from induced resistance, and are the consequence of a more subtle direct physiological effect in constraining leaf expansion.