The Chilean forest industry represents the second source of incomes, after copper, where Pinus radiata is the major cultivated species; this production level has generated new alternatives searching to increase the quality of pine seedlings. The use of organic sources of fertilization and the use of fungi, like Trichoderma harzianum, arise as production alternatives. This study evaluated plant growth stimulation activity of native strain of T. harzianum on Pinus radiata seedling and the effect of compost as substratum in the interaction of these organisms, both plant (fitness) and fungus (population). The results indicated a positive significant interaction between T. harzianum and compost over height and biomass of the plants, as well as in the development of the radical system, by the combination of compost and T. harzianum. Compost increases the size and population rate of T. harzianum. Compost substratum inoculated with T. harzianum increases significantly the fitness of P. radiata seedling. The involved mechanisms have not been elucidated.