Red fescue (Festuca rubra) is a perennial grass commonly infected by the fungal endophyte Epichloe festucae in several different European habitats. In a greenhouse experiment, F. rubra plants with different endophyte status (E+ = infected; E = non-infected) were grown in mono-cultures and in simple mixture with red clover (Trifolium pratense). The purpose of the study was to determine the effect of endophyte infection on the competition of red fescue plants with red clover. Root and shoot growth of red clover was significantly depressed when grown in competition with E+ plants compared to E plants. When grown in mono-culture or in competition with red clover, root biomass of E+ red fescue plants was lower than that of E- plants, but no differences in shoot biomass between E+ and E- were found. No differences in red fescue plant biomass between growth in monoculture and with red clover were found. The lower root-biomass of infected plants and the reduction of red clover growth suggest that infected plants could release chemical compounds that reduce legume growth, that is, an allelopathic interaction.