Decomposition was influenced by wood moisture content (MC), with decomposition rates estimated at 0.2, 0.9. and 1.6 percent per week of the total bound methylisothio-cyanate (MIT) for wood fumigated at 0, 12, and 60 percent MC, respectively. Dimethylthiourea and 2,4-dimethyl-1,2,4-thiadiazolidine-3,5-dithione, which formed during fumigation, showed toxic activity against the decay fungus Poria carbonica. Elemental sulfur was also formed, but showed minimal fungitoxic activity. Some MIT remained in fumigated wood even after extensive aeration under dry conditions. This residual MIT rapidly volatilized at fungitoxic concentrations when wood was wetted and may provide residual protection against fungal invasion.