The glucose oxidase gene from the biocontrol fungus Talaromyces flavus has been isolated and shown to be only 64% identical at the amino-acid sequence level to the similar enzyme from Aspergillus niger. A transformation system has been developed for both T. flavus and the related T. macrosporus and has been used to create Talaromyces spp. which either over-express or are deficient in glucose oxidase. In vitro inhibition experiments on Verticillium dahliae using culture filtrates from these transformants indicates that secreted glucose oxidase is responsible for a large part of the growth inhibition of V. dahliae microsclerotia and hyphae by T. flavus, although other inhibitory compounds may also play a role. In pot trials with cotton plants, both Talaromyces species had some biocontrol activity, but there was no significant difference in the incidence of Verticillium wilt with either the presence or absence of glucose oxidase activity in the biocontrol fungus. Under the experimental conditions used, insufficient glucose is presumably present in the soil around cotton roots to generate sufficient hydrogen peroxide to inhibit V. dahliae and the observed biocontrol activity must be attributed to other factors.