Ammonium fumarate production from glucose-based media by Rhizopus arrhizus NRRL 1526 with mycelial growth controlled by phosphorus limitation exhibited mixed-growth-associated product formation kinetics, with growth-associated production related to secondary mycelial growth only. The contribution of the primary mycelial growth phase was minimised by resorting to prolonged batch production using free mycelia under intermittent glucose feeding or repeated batch production using immobilised mycelia. The metabolic activity of free or immobilised mycelia was limited by fumarate accumulation or by oxygen diffusion phenomena, respectively. For batch cultures in a 15 dm(3) stirred bioreactor the peripheral impeller speed (nu(I)) was increased from 1.88 to 3.3 ms(-1), and the fumarate yield coefficient on glucose increased from 0.25 +/- 0.01 to 0.42 +/- 0.02 g g(-1), while the malate yield coefficient on fumarate (Y-M/F) reduced from 0.46 +/- 0.01 to 0.14 +/- 0.01 g g(-1). With a net increase in the fumarate-to-malate ratio from 2 to 6.5, a nu(I) value of 3.3 ms(-1) gave the best fermentation performance and provided a basis for further scale-up studies.