Response surface modeling was applied to shake-flask cultures of Pachysolen tannophilus, CBS 4044, to determine the optimum temperature and pH for growth. A two-factorial experiment was designed and the results were analyzed and plotted using MODDE software. This technique proved efficient at identifying the optimum temperature and pH by performing a limited number of fermentations while varying the two variables at the same time. The results indicated that the maximum yield of biomass density corresponded to pH 3.7 and a temperature of 31.5 degrees C, at an initial carbon source (D-xylose) concentration of approximately 50 g l(-1).