Gliocladium virens, a mycoparasite of Rhizotonia solani, acts as a biocontrol agent of cotton damping-off and soreshin when an air dried peat moss-czapek's broth (PMCZB) preparation of the antagonist is added with cottonseed at planting. However, the failure of some strongly parasitic strains to function well as biocontrol agents has led to questions about the importance of mycoparasitism in the biocontrol process. Therefore, effective strains of G. virens were irradiated with ultraviolet light, and mutants with no mycaparasitic activity were isolated. Comparison of PMCZB cultures of parent and mutant strains as cottonseed treatments, or as antagonists of R. solarii sclerotia in natural soil, showed both parent strains and mycoparasitic deficient mutants equally capable of disease control and of killing pathogen sclerotia in soil. These results indicate that mycoparasitism is not an important mechanism in the biocontrol of R. solani initiated cotton seedling disease by G. vires.