We report the results of a study investigating the influence of elevated CO sub(2) on species interactions across three trophic levels: a plant (Brassica oleracea ), two aphid herbivores (the generalist Myzus persicae and the specialist Brevicoryne brassicae ), and two natural enemies (the coccinellid Hippodamia convergens (ladybird) and the parasitoid wasp Diaeretiella rapae ). Brassica oleracea plants reared under elevated CO sub(2) conditions (650 ppmv vs. 350 ppmv) were larger and had decreased water and nitrogen content. Brevicoryne brassicae reared on plants grown in elevated CO sub(2) were larger and accumulated more fat, while there was no change in M. persicae traits. Fecundity of individual aphids appeared to be increased when reared on plants grown in elevated CO sub(2) . However, these differences were generally lost when aphids were reared in colonies, suggesting that such changes in plant quality will have subtle effects on aphid intraspecific interactions. Nevertheless, CO sub(2) treatment did influence aphid distribution on plants, with significantly fewer M. persicae found on the shoots, and B. brassicae was only found on senescing leaves, when colonies were reared on plants grown in elevated CO sub(2) . We reared B. brassicae and M. persicae in competition on plants grown at both the CO sub(2) concentration treatments. We found a significantly lower ratio of M. persicae : B. brassicae on plants grown under elevated CO sub(2) conditions, strongly suggesting that increasing CO sub(2) concentrations can alter the outcome of competition among insect herbivores. This was also reflected in the distribution of the aphids on the plants. While the CO sub(2) treatment did not influence where B. brassicae were found, fewer M. persicae were present on senescing leaves under elevated CO sub(2) conditions. Changes in plant quality resulting from the CO sub(2) treatments did not appear to alter aphid quality as prey species, as the number consumed by the ladybird H. convergens , and the number parasitised by the parasitoid wasp D. rapae , did not change. To our knowledge, this study provides the first empirical evidence that changes in host plant quality mediated by increasing levels of CO sub(2) can alter the outcome of interspecific competition among insect herbivores. none