Presence of Trichoderma aggressivum f. aggressivum (green mold disease) either in compost or casing soil causes significant yield losses in button mushroom (Agaricus bisporus) cultivation. The aim of this study was to examine biotic interactions between A. bisporus and T. aggressivum f. aggressivum in compost and casing soil. In the study, strains (brown and white) of A. bisporus and isolates (Şem1/1 and K3) of T. aggressivum f. aggressivum were used. Experiments (compost and casing soil) were conducted according to completely randomized design in factorial with four replications. Yield values of each experiment were established by weighing sporophores (mushrooms) in each plot. Mean yield values of each treatment were compared with others. Presence of the T. aggressivum f. aggressivum isolates in compost and casing soil significantly (P˂0.01) inhibited mycelial growth and fruiting body formation of both strains of A. bisporus. In the presence of the T. aggressivum f. aggressivum isolates in casing soil, average yield loss of the strains of A. bisporus was 36.41%, while it was 29.35% in the presence of the T. aggressivum f. aggressivum isolates in compost. Average yield loss of the white strain of A. bisporus was 49.68%, whereas it was 16.08% in the brown strain. The study revealed that presence of T. aggressivum f. aggressivum in casing soil could be much more negative influence on yield of A. bisporus than that of compost and brown strain of A. bisporus could be more resistant to T. aggressivum f. aggressivum than white strain.