Insect gut microbes have been considered as a resource for bioactive metabolites. The aim of this study was to characterize the compounds of a fungus Aspergillus terreus QT122 associated with the gut of dragonfly. Five main phytotoxic, antifungal, and immunosuppressive substances were isolated from the fungus QT122. The structures of such compounds were identified as emodin (1), 1-methyl emodin (2), terrein (3), methyl 6-acetyl-4-methoxy-7,8-dihydroxynaphthalene-2-carboxylate (4), and dihydrogeodin (5) on the basis of spectroscopic analysis and by comparison of the corresponding data to those reported in the literature previously. The compound 3 exhibited the best phytotoxic activity against the radicle growth of A. retroflexus L. and E. crusgalli L. with their IC50 values of 11.2 and 3.1 mu g/mL, which were comparable to that of the positive control of 2,4-dichlorophenoxyacetic acid (2,4-D) with the IC50 values of 8.1 and 1.6 mu g/mL, respectively. The compounds 2-3 showed potent antifungal activity in the growth of Alternaria solani with the IC50 value of less than 0.1 mu g/mL and the compound 2 also had great inhibitory effect against the growth of Fusarium oxysporum f. sp. cucumerinum (IC50 < 0.1 mu g/mL), which was comparable to that of referenced cycloheximide with IC50 value of below 0.1 mu g/mL. The compounds 3-5 exhibited strong immunosuppressive activities against the T cell viability with the inhibition rates of more than 99%, which were comparable to positive cyclosporin A under the concentration of 20 mu M. These results suggest that the compounds 2-5 have the potential to be used as bio-control agents in agriculture or immunosuppressive agents.