L(+)-lactic acid and its salts are being widely used in food and pharmaceutical industries. Rhizopus oryzae is known to be outstanding in producing optically pure L(+)-lactic acid. Rhizopus cultures are morphologically complex. They can grow as extended filamentous form, mycelial mats, mycelial pellets or clumps. Different morphology growth forms can have a significant effect on the rheology of the fermentation broth that will affect the performance of the bioreactor. The factors that affect the morphology and L(+)-lactic acid productivity of R. oryzae R1021 were investigated in detail in this work. Studies on the relationship among CaCO3 addition, inoculum size, concentration of NH4NO3, agitation speed, aeration rate and the morphology of the mycelial growth and lactic acid production were carried out. Uniform sized mycelial pellet growth (diameter of 1.0-2.0 mm) was obtained when the time of adding CaCO3 is 8 h after fermentation. The pellet form exhibited a higher lactic acid yield than clump form. An inoculated spore concentration of 106 spores/ml induced uniform pellet in preculture medium, and then formed the small mycelial pellets in production medium and a maximum L(+)-lactic acid concentration of 72.62 g/l was observed at the inoculum level of 3 x 106 spores/ml. Further studies on the effect of agitation speed on the fungal morphology showed that increasing agitation speed caused the fungal pellets to decrease in size but to increase in number per unit volume. Similar studies on the effect of aeration rate on the fungal morphology showed that increasing aeration rate caused the fungal pellets to increase in size but to decrease in number per unit volume. When agitation speed and aeration rate were 300 r/min and 0.6 vvm, the final L(+)-lactic acid concentration and the biomass concentration are 76.1 and 9.62 g/l, respectively, after 36 h culture of R. oryzae R1021 in pellet form (average pellet size, 1.4 mm) in stirred tank fermentor with initial glucose concentration of 100 g/l. L(+)-lactic acid production and the yield Of L(+)-lactic acid based on the initial glucose are above 74.92 g/l and 74.2%, respectively, in 6 batches repetitive fermentation. (C) 2003 Elsevier Science Ltd. All rights reserved.