The influence of water potential on linear mycelial growth, secretion, and the in vitro activities of enzymes beta-glucosidase, cellobiohydrolase, beta-xylosidase, exochitinase, and chymotrypsin of Trichoderma harzianum strain T66 was studied at different temperatures. Nearly linear correlation was found between water potential and colony growth rate at both 25 degrees C and 10 degrees C, with higher growth rates at the higher temperature and higher water potentials. The amounts of enzyme secretion depended on the water potential and not on the quality of salt (NaCl or KCl) used as osmoticum, Enzyme activities were significantly affected by water potential. Significant enzyme activities were measured for most of the enzymes even at - 14.800 megapascal (MPa), which is below the water potential where mycelial growth ceased. These results suggest the possibility of using mutants with improved xerotolerance for biocontrol purposes in soils with lower water potentials.