All organisms share similar mechanisms in the heat shock response, such as synthesis of conserved heat shock proteins. Here, we report on the cloning, characterization and functional analysis of a Trichoderma harzianum T34 hsp70 gene. The expression of this gene was evaluated in cultures grown in abiotic stress conditions. An increased level of expression was detected when the fungus was grown at 37 or 41 degrees C, as well as in the presence of oxidative or osmotic agents. The overexpression of hsp70 in T. harzianum T34 gave rise to transformants with higher quantities of biomass obtained after heat shock treatment. In addition, these transformants showed an enhanced tolerance to oxidative, osmotic and salt stresses when conidia were previously treated at 45 degrees C for 2h.