It is desirable to modify the normally filamentous Trichoderma reesei Rut C-30 to a pellet form, for easy biomass separation from the fermentation medium containing soluble products (e.g., cellulase). It was found in this study that this morphological modification could be successfully achieved by addition of the biosurfactant rhamnolipid (at >= 0.3 g/L) and the synthetic Triton X-100 (at >= 0.1 g/L) to the fermentation broth before the cells started to grow actively. Thirteen other surfactants tested were not as effective. Furthermore, the added rhamnolipid and Triton X-100 increased the maximum cellulase activity (Filter Paper Units) produced in the fungal fermentation; the increase was 68 +/- 7.8% for rhamnolipid and 73 +/- 12% for Triton X-100. At the concentrations required for pellet formation, rhamnolipid had negative effect on the cell growth: with increasing rhamnolipid concentrations, the growth rate decreased and the lag-phase duration increased linearly. Triton X-100 caused no significant differences in growth rate or lag phase. (C) 2012 Elsevier Inc. All rights reserved.