none In order to understand the role that interspecific competition plays in phytophagous insect communities, we need to consider the ways in which the nature of competition between species can be complicated by interactions with members of other trophic levels. In this paper, I describe how the competitive interaction between two species of gall forming fly, Lipara lucens and L. rufitarsis, is modified as a consequence of interactions with a host plant and natural enemies. The results of field experiments and observations of wild populations show that the two species compete strongly for stems of the common reed, Phragmites australis. L. lucens is competitively inferior, having significantly increased mortality in the presence of heterospecifics. No lasting negative effects of interspecific competition on L. rufitarsis could be found. In fact, the results show that L. rufitarsis can benefit from the presence of heterospecifics. It can usurp the galls of L. lucens, and experiences reduced mortality and increased fecundity as a result. Thus, L. rufitarsis can be described as a kleptoparasite. Kleptoparasitism in this system is mediated by the host plant and by the principal parasitoid of L. rufitarsis. L. rufitarsis can only benefit from the presence of its competitor because L. lucens induces P. australis to form a large, robust gall that repels the attacks of the ichneumonid wasp Endromopoda phragmitidis. These results show how facultative kleptoparasitism can modify the ecological consequences of interspecific competition for both individuals and populations. Whole animal physiology; Nutrition; Feeding behaviour; Foraging; Parasites diseases and disorders; Hosts; Ecology; Population dynamics; Competition; Land zones; Palaearctic region; Eurasia; United Kingdom; Europe Lipara lucens (Chloropidae); Physiological condition; Fitness; Plant hosts; Phragmites australis; Survival; Interspecific competition, fitness & survival consequences; England; Cambridgeshire, interactions with host plant & natural enemies effects, consequences Lipara rufitarsis (Chloropidae); Physiological condition; Fitness; Food robbing; Intraguild kleptoparasitism; Causes & consequences; Plant hosts; Phragmites australis; Survival; Interspecific competition, fitness & survival consequences; England; Cambridgeshire, interactions with host plant & natural enemies effects, consequences