The only fungal antagonists that have received serious attention as potential biocontrol agents of cotton diseases are Trichoderma harzianum (Th) and Gliocladium virens (Gv). Both of these fungi are similar in appearance on agar media, and they have many characteristics in common (3). They are fast growing Hyphomycetes that produce green conidia, and both produce thick walled chlamydospores in culture. They are also very aggressive mycoparasites that attack and parasitize the hyphae and resting structures of other fungi. These similarities have caused them to be mistaken for one another in the literature, and many of the features of G. virens have been erroneously ascribed to Trichoderma spp. (1, 13, 14). The two biocontrol agents differ from one another in that the conidia of Th are borne on the conidiophore in a dry fashion while those of Gv are borne in a water droplet at the apex. They also differ in that Gv produces many more antibiotic and bioactive secondary metabolites than does Th. Of the Trichoderma spp., Th appears to be the most effective as a biocontrol agent. As early as 1982, researchers in Israel (4) reported the control of Rhizoctonia solani on cotton by coating the seed with conidia of T. harzianum. This fungus has also been reported to control cotton seedling disease incited by Pythium ultimum (6), but solid matrix priming of the seed was necessary for effective control. In the past, field testing with this biocontrol agent has shown that its performance is quite variable and often less effective than the chemical control. However, field tests across the cotton belt with an Oklahoma isolate have given encouraging results in the last three years. Work with Gv as a biocontrol agent of cotton seedling diseases has been more extensive and more successful than that with Th, and the balance of this paper will, therefore, be focused on Gv.