Fungicides can have a good result against phytopathogens, but alternatives, such as a biological control agents, have also been found to be effective and ecofriendly. In this experiment, Alternaria solani was isolated from rotten tomato fruit. The culture filtrates of Trichoderma harzianum gradually inhibited the radial growth of A. solani at higher concentrations, but growth was not completely inhibited until a high filtrate percentage of 75% was reached (75% by volume). The microscopic study revealed that the T. harzianum culture filtrate and its spores changed the size, number, and shape of the A. solani conidiospores. A. solani produced both cellulolytic and pectinolytic enzymes in vitro. The activity of these enzymes decreased with an increase in the T. harzianum filtrate percentage and chemical fungicide concentration. A. solani failed to produce hydrolytic enzymes, particularly pectinase, at high concentrations of fungicide (100 ppm and 150 ppm). When T. harzianum was used as a biocontrol agent, the detected hydrolytic enzyme activities increased compared with the other treatments.