Aims: To purify and characterize an extracellular α-glucosidase from Trichoderma viride capable of inactivating a host-specific phytotoxin, designated RS toxin, produced by the rice sheath blight pathogen, Rhizoctonia solani Kühn. Methods and Results: The host-specific RS toxin was purified from both culture filtrates (culture filtrate toxin, CFTox) and R. solani-inoculated rice sheaths (sheath blight toxin, SBTox). Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analyses of extracellular proteins, purified from a biocontrol fungus T. viride (TvMNT7) grown on SBTox and CFTox separately, were carried out. The antifungal activity of the purified high molecular weight protein (110 kDa) was studied against RS toxin as well as on the sclerotial germination and mycelial growth of R. solani. Enzyme assay and Western blot analysis with the antirabbit TvMNT7 110-kDa protein indicated that the protein was an α-glucosidase. The 110-kDa protein was highly specific to RS toxin and its Michaelis-Menten constant value was 0·40 mmol 1-1 when p-nitrophenyl α-D-glucopyranoside was used as the substrate. The isoelectric point of the protein was 5·2. N-terminal sequencing of the α-glucosidase protein showed that its amino acid sequence showed no homology with other known α-glucosidases. Conclusions: This appears to be the first report of the purification and characterization of an α-glucosidase capable of inactivating a host-specific toxin of fungal origin. The α-glucosidase is specific to RS toxin and is different from the known α-glucosidases. Significance and Impact of the Study: As RS toxin could be inactivated by the microbial α-glucosidase enzyme, isolation of the gene that codes for the enzyme from T. viride and transfer of the gene to rice plants would lead to enhanced resistance against sheath blight pathogen by inactivation of RS toxin.