ABSTRACTMeasurements of competitive saprophytic colonization were used to evaluate the activity ofTrichoderma harzianumin non-sterile soil under conditions of varying soil moisture and temperature. Estimates of activity based on levels of colonization, determined after homogenization of substrate pieces, were more discriminating than those based on the number of colonized pieces.T. harzianumwas found to be most active at soil matric potentials from -0.5 to -1.0 bar, with activity declining at matric potentials of 0.0, -7.5, and -15.0 bars. The saprophytic activity ofT. harzianumwas greatest at temperatures from 15 to 21 C. The optimum temperature for competitive colonization was several degrees lower than the optimum temperature for growthin vitro.