Abstract: Mutant LR which is fast growing and grows at low temperature (10 ℃) , and mutant LRR which is resistant to carbendazim (20μg/ ml) , were obtained f rom a Trichoderma viride st rain L TR-2 by ult raviolet mutation. The two mutant s are weaker in inhibiting the pathogens in vitro against cotton pathogens ( Fusari um oxysporum f . sp. vasinfect um , Rhizoctonia solani , and V erticillium dahliae) compared to parental st rain. LR is more adapted in non-rhizosphere soils , and LRR is less competent in healthy cotton rhizosphere than L TR-2. LR is not effective in cont rolling cotton damping-off caused by R . solani , but is more effective than L TR-2 in cont rolling both cotton wilt dseases caused by F. ox ysporum f . sp. vasinfectum and V . dahliae , respectively. LRR is similar to L TR-2 in cont rolling all the three diseases.