Direct ammonium fumarate production from glucose-based media with Rhizopus arrhizus NRRL 1526 was obtained using 2 kmol (NH4)(2)CO3 per m(3) as neutralising agent and controlling mycelial growth by phosphorous (P) limitation. As the P level in the production medium was increased from 0 to 0.3 kg of KH2PO4 per m(3), the fumarate yield decreased from 0.32 to 0.13 g per g of glucose consumed; maximum ammonium fumarate productivity (0.46 kg m(-3) h(-1)) was obtained when using 0.1 kg phosphate m(-3).