The morphology of Monascus cells was controlled and pigment fermentations scaled up from a 5 1 jar to a 300 1 pilot vessel. At a rotational speed of 350 rpm or less, serious mycelial aggregation was present with pigment yields of approximately 37.5 OD units. Monascus cells with long hairy mycelia were induced, resulting in viscous cultures. As the rotational speed increased from 350 to 700 rpm, the k(L)a values increased gradually from 0.003 to 0.029 s(-1). A maximum pigment yield of 220 OD units was obtained at 500 rpm where mycelia with short branches were induced. The specific oxygen uptake rates (q(O2)) at 600 and 700 rpm were also lower than the rate at 500 rpm.. Mycelia had short branches and appeared to be damaged by mechanical shear forces due to high agitation speeds. When seed cultures were grown in jar fermenters under high shear stress, the pigment yields and DOT values of fermentations were also high. Fermentations were successfully scaled up from 5 1 jars to a 300 1 pilot vessel based on a constant I impeller speed of 12,500 cm min(-1), which was the best condition for pigment production and cell morphology. (C) 2002 Elsevier Science Ltd. All rights reserved.