This study investigated the effects of elevated CO2 on populations of root and/or shoot aphids and their effects on partitioning in Cardamine pratensis. Total plant biomass in elevated ( 600 ppm) CO2 of uninfested Cardamine plants was 52% higher than in ambient ( 350 ppm) concentrations but CO2 effects were not statistically significant. In elevated CO2, feeding by shoot aphids (Aphis fabae fabae) alone and in combination with root aphids (Pemphigus populitransversus), and root aphids alone had no significant effect on plant biomass. No significant effects of elevated CO2 were detected on population size of the shoot or root-feeding species. Interspecific effects were detected between the root and shoot species. Root aphid populations were significantly smaller in the presence of shoot aphids on the same plants. In this system plant growth was unaffected by an elevated CO2 environment. Plant species which are more sensitive to elevated CO2 may show a modified response to herbivore pressure in a future atmospheric environment.