Numerous fungi and bacteria, including existing biocontrol strains with known activity against soilborne fungal pathogens, were tested for their efficacy in controlling Fusarium wilt diseases. Specific nonpathogenic strains of Fusarium spp., isolated from wilt-suppressive soils, were the most effective antagonists for the reduction of Fusarium wilt diseases of tomato, watermelon, and muskmelon, providing consistent and significant disease control (50-80% reduction of disease) in several repeated tests. Other organisms, including isolates of Gliocladium virens and Trichoderma hamatum also significantly reduced Fusarium wilt compared to disease controls (30-60% reduction). Ecological studies determined that induced systemic resistance is involved in the mechanism of action for selected nonpathogenic Fusarium antagonists, and that these antagonists are effective at low inoculum densities, at high pathogen densities, and under a variety of different environmental conditions. These isolates have potential for development as biological control agents for the control of Fusarium wilt diseases of numerous crops. The fungal antagonist Talaromyces flavus has potential for the biological control of Verticillium wilt diseases. Production of the extracellular enzyme glucose oxidase, and the subsequent release of hydrogen peroxide, is associated with biocontrol efficacy. Improved control by this antagonist may be possible through combinations with sublethal heat or fumigation treatments, which weaken the pathogen and make it more susceptible to the action of T. flavus.