Solid state fermentations were carried out using a strain of Rhizopus formosa MUCL 28422 for its ability to grow on raw cassava. Optimal substrate composition, as estimated by surface response design experiments, was 10% each of cassava bagasse and soybean flour and 80% cassava flour. Optimal fermentation conditions were: temperature 32 degrees C, initial pH and moisture of the substrate 6.5 and 64%, respectively, and inoculum size 10(6)-spores g(-1) dry matter. These conditions were employed for studying the kinetics of the biotransformation of cassava flour using Oxygen Uptake Rate (OUR) and Carbon dioxide Evolution Rate (CER) during the process. The respiratory quotient was similar to 1, corresponding to an aerobic process in the first 24-26 h. Sporulation started after 26 h of fermentation when the respiratory quotient showed a trend to increase up to similar to 6, Biomass estimation, based on oxygen consumption (Y-x/o) gave a value of 0.531 g biomass g(-1) O-2 consumed and the maintenance coefficient (m) was of the order of 0.068 g O-2 consumed g(-1) biomass h(-1). The corresponding value for the specific growth rate (mu) was 0.16 h(-1).