In general, high broth viscosity is a key factor to be considered in a submerged fermentation of filamentous fungi. High broth viscosity was also observed in a batch fermentation of Monascus sp. J101 at 30 degrees C. In a batch culture at 30 degrees C, most cell growth was accomplished within 48 h, which induced highly entangled clumps. The resultant high viscosity induced heterogeneity inside the fermentor, poor oxygen transfer, and low pigment yield. However, these problems could be overcome by reducing fungal growth rate through culture at low temperature (25 degrees C). Cell growth was moderate and continued for 120 h, and low viscosity was maintained. The DO levels remained at 50% or higher with good mixing. As a result, the pigment yield at 25 degrees C was 10 times greater than at 30 degrees C.