Statistical designs were used to optimize some parameters affecting the growth rate of a Brazilian strain of Thermoascus aurantiacus. The mycelial growth rate was measured using the horizontal tube method. Temperature of incubation and initial pH were the major factors affecting the growth rate, They were optimal at 6.0 and 48 degrees C, respectively. The maximum growth rate was obtained in solid Czapek modified medium containing 1.5% glucose and 38.4 mEq L(-1) NaNO3. Under these conditions, the growth rate of T. aurantiacus was 5.16 +/- 0.10 mm h(-1). Lignin-related compounds such as tannins and extractive substances added at 0.1% (w/v) to the minimal Czapek medium increased growth rate 14% and 29%, respectively.164224229