Abstract: The effect of continuous exposure to 400, 800 and 1200ppm CO2 on growthparametres of tomato seedlings and on fecundity and longevity of Trialeurodes vaporariorum(Westwood) was studied in growth chambers. Root/shoot ratio of tomato decreased withincreasing CO2 concentration, but was not affected by insect feeding. Stem length wassignificantly affected by both elevated CO2 and insect feeding, but not by their interaction whichmeans whiteflies did not substantially modify the response of their host plants to elevated CO2.C/N-ratio of leaves increased with increasing CO2 level. CO2 treatment significantly reduced totalnumber of eggs laid per female but not the female longevity. Whitefly fecundity thus decreasedtogether with the significantly increasing C/N ratio, which may indicate diminishing plant qualityfor greenhouse whiteflies by the elevated CO2.