1 Competition for nutrients is an important factor structuring plant communities. Plant symbionts such as arbuscular mycorrhizal fungi (AMF) can have considerable influence on nutrient uptake and are therefore likely to influence plant competition. In this study we investigated the influence of different AMF isolates on the competitive relationships between a legume (Lotus corniculatus L.) and a grass (Festuca ovina L.), and between L. corniculatus and a forb (Plantago lanceolata L.). 2 AMF altered the competitive interactions between the investigated plant species. The legume, which was the most AMF-dependent plant species, was favoured in the presence of AMF relative to the non-AMF treatment in seven out of eight cases. Competition between the grass and the legume was the most strongly affected by AMF. The grass and the legume both benefited from AMF when grown alone, but in competition AMF favoured the legume and grass biomass was reduced by 38%. 3 The presence and also the identity of AMF influenced the competitive relationships. The extent to which AMF influenced plant competition and changed the relative abundances of the competitors depended on the specific AMF isolate. 4 This study shows that both presence and identity of AMF can influence plant competition. These results indicate that AMF and the composition of AMF communities regulate plant interactions and determine the structure of plant communities. © 2007 The Authors.