Gremmeniella abietina (Lagerberg) Morelet is a pathogenic fungus that causes severe damage in coniferous forests, causing the death of the trees, in Central and Northern Europe, North America and Japan. Biological control (i.e. the use of biologically antagonistic organisms) is being considered as an alternative and an eco-friendly method to deal with plant diseases. Among such organisms several fungal endophytes have been successfully used to reduce or inhibit the growth of pathogens. Thus, the aim of this study was to evaluate the ability of several fungal endophytes to reduce the G. abietina spreading on pine seedlings, with the goal of exploring the mechanisms involved in that reduction. The experiment was carried out on two-year old P. halepensis seedlings under greenhouse conditions. Five fungal endophytes (Trichoderma spp., Aureobasidium pullulans, Aureobasidium spp., endophyte 20.1 and Leotiomycete spp.) obtained from healthy P. halepensis trees were used to evaluate their effect on six G. abietina isolates. The pathogen and the endophyte were both inoculated in every seedling. At the end of the experiment seedlings were cut and brought to the laboratory where the necrosis length and total phenol content of the plant were measured. The pathogen presence was determined by a nested PCR with specific primers of G. abietina. The presence of all endophytes significantly reduced the necrosis length caused by G. abietina in most of the cases. However, the phenolic content of the plant, which is an indicator of the activation of the plant's defence mechanisms, had not increased with the endophyte inoculation. Therefore, the reduction in the necrosis observed could be attributed to a direct effect of the endophyte on the Gremmeniella spreading. These results suggest that the use of fungal endophytes could be an effective way to protect against G. abietina infections.