Viridiol(-) mutants of the biocontrol agent Trichoderma virens can no longer synthesize the phytotoxin viridiol, but retain the capacity to produce antifungal antibiotics, act as mycoparasites, and control cotton seedling disease incited by Rhizoctonia solani. The use of biocontrol preparations containing viridiol (-) mutants virtually eliminated the phytotoxicity associated with treatment of cotton seed with high concentrations of parent strain preparations. The phytotoxic and biocontrol activities of parent and viridiol (-) strains were compared by coating Apron treated cotton seed with a latex sticker and air-dry, finely ground, preparations of the fungi. Field tests of treated seed in plots heavily infested with seedling disease pathogens at Shafter and Dos Palos, California showed that treatment with parent or some mutant strains of T. virens gave significantly better stands than nontreated or fungicide treated controls. With the exception of one strain, all seed treated with the biocontrol agents gave higher average lint yields than the nontreated or Apron treated controls, although the differences were not significant.