Practical use of Trichoderma harzianum requires feasible formulated products. The objective of this study was to investigate the survival and effectiveness of a T. harzianum bentonite–vermiculite formulation against fusarium wilt and its growth-promotion effect on melon plants under nursery conditions compared with the incorporation of this agent as a nonformulated conidia suspension. The effectiveness of the application of T. harzianum to melon plants was related directly to its formulation, as the formulation had a clear influence on its survival on peat. The formulated form maintained the inoculation level of T. harzianum colony-forming units after 8 weeks, whereas the nonformulated form reduced it by two orders of magnitude. Plants treated with the bentonite–vermiculite formulation showed a higher shoot weight and higher resistance to fusarium wilt disease.