The mechanism of the exclusive growth of Termitomyces in fungus combs with fungi-growing termites, O. formosanus was examined using laboratory scale fungus combs. In the combs without the termites, vigorous growth of unidentified fungi was observed although no significant change was found in the case of the combs with termites. In addition, these results were reproducible even when incubated in a separated dish, suggesting that the physicochemical conditions were not the reason for the growth. With the molecular based analysis for the microbial communities in the combs, monoculture of the Termitomyces in the combs with termites was confirmed while the bacterial communities were independent either with or without termites. Possible mechanism of the exclusive growth of Termitomyces, such as the selective grazing of pathogenic fungi or contribution of antifungal activity giving actinomycetes were also discussed.