The endochitinase gene ech42 from Trichoderma harzianum was cloned and expressed in Escherichia coli BL21(D3)pLysS using a pET-19b expression system. The effect of temperature, cell concentrations and inductor concentration on protein expression were evaluated by response surface methodology. Applying a second-order model to describe the amount of endochitinase produced by two selected clones, demonstrated that different optimal conditions of expression were found among clones, indicating that the best environmental conditions for expression among the several isogenic clones differed. (C) 2003 Elsevier Ltd. All rights reserved.