Image analysis was used to measure the effect of Tween 40 on the morphology of Trichoderma harzianum. The percentage of pellets was maximal (93%) with Tween at 0.2 ml l(-1), whereas the maximal proportion of dispersed mycelia (40%) was with Tween at 1.6 ml l(-1). The particle median diameter decreased from 2 to 0.5 mm as Tween concentration was increased and was inversely correlated to the biomass dry weight. Adding pre-determined aliquots of Tween 40 to the culture medium can be used to define the morphological characteristics of Trichoderma harzianum in submerged shake-flask culture.