Protoplasts were isolated from Trichoderma harzianum strain PTh18 using lysing enzymes and self-fusion of T harzianum protoplasts was carried out using polyethylene glycol in STC buffer. The fused protoplasts of T harzianum were regenerated and 15 self-fusants were selected to study the chitinase production and biocontrol activity. High chitinase activity was measured in the culture filtrates of most of the self-fusants (87%) than the parent. Among the fusants, the strain SFTh8 produced maximum chitinase with a two-fold increase as compared to the parent strain. All the self-fasants exhibited increased antagonistic activity against Rhizoctonia solani than the parent. The crude chitinase preparation of SFTh8 lysed the mycelia of T harzianum, Trichoderma viride and Trichoderma reesei and released the protoplasts in higher number than the crude chitinase preparation of parent strain PTh18. (c) 2005 Elsevier Ltd. All rights reserved.