The present study explores the insect toxicity and antifungal activity of 2, 3-dimethylmaleic anhydride against mixed-age cultures of several stored product insects namely: the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae); the pulse beetle, Callosobruchus chinensis Fab (Coleoptera: Bruchidae) and the rust-red flour beetle, Tribolium castaneum Herbst (Coleoptera: Tenebrionidae) and plant fungal pathogens namely: Fusarium oxysporum f. sp. cepa (Hypocreales:Nectriaceae) and Sclerotium oryzae Cattaneo (Magnaporthales: Magnaporthaceae), respectively. The study also revealed that the mixed-age-cultures of C. chinensis were slightly tolerant to 2, 3-dimethylmaleic anhydride, showing 80–90% mortality in 72-h exposure at 100 µg/L dose only. Whereas mixed-age-population of S. oryzae and T. castaneum were fairly susceptible, attaining 100% population extinction at 100 µg/L doses of 2, 3-dimethylmaleic anhydride in 24, 48 or 72 h of exposure. Further, the isolated compound also inhibits mycelia growth of the two filamentous fungi under study at 5 mg/plate and inhibition was dose-dependent and species-specific. This is first to report showing the antifungal activities of 2, 3-dimethylmaleic anhydride. Finally, results reveal that 2, 3-dimethylmaleic anhydride could be potential biopesticides for plant fungal pathogens and stored product beetles.