Because of the importance of herbivory in many herbaceous plant–dominated ecosystems, it will be important to determine whether interactions exist between herbivory and CO2 enrichment if we are to understand better how ecosystems will respond to global change. In an experiment with the C3 grasses Poa pratensis and Phleum pratense, plants were grown together under ambient (360 ppm) and elevated (650 ppm) CO2, with and without aboveground defoliation, and with or without additions of the root-feeding nematode Pratylenchus penetrans. The objective was to determine whether herbivory would interact with CO2 enrichment in determining biomass responses or whether these factors were additive. Responses to CO2 enrichment were different between species. Poa root and shoot biomasses were unresponsive to CO2 enrichment, whereas Phleum showed a large increase in aboveground biomass and productivity in response to CO2 enrichment. As a result, Phleum was much more dominant over Poa in mixture under elevated CO2 than under ambient CO2. Elevated CO2 tended to dampen the increases in productivity associated with defoliation, but this interaction was not quite significant (Pp 0.07). Aboveground biomass and productivity were not significantly reduced by nematode additions. Root biomass ( ) declined with high nematode infestation rates, and this decline was greater with CO2 Phleum+ Poa enrichment and without defoliation.