Fumaric acid was produced in repeated batch processes using Rhizopus arrhizus NRRL 1526 immobilized in polyurethane sponge particles and cultivated in a fluidized-bed reactor Fermentation conditions were established for concentration of carrier-particles and optimal concentrations of glucose molasses (as glucose equivalent) and ammonium sulphate. Acid production was optimized (12.3 g/litre, 35.56% and 0.256 g/litre h of fumaric acid accumulation, yield and volumetric productivity of the whole process, respectively) under the following conditions : carrier-particle concentration, 20% (v/v) of the working volume; glucose molasses, 50 g/litre; ammonium sulphate, alternation of cycles with 0.0 and 0.5 g/litre. The rime of fermentation was 48 h per batch for a total of eight batches. Scanning electron microscopy revealed that the fungus began to grow within the particle cells but soon after the end of the first batch growth was essentially confined to the outer part of the polyurethane particle, where a peripheral layer of mycelium began to form.