The extracellular production of Escherichia coli phytase was studied in fed-batch fermentations. Two different feeding strategies were compared: control by keeping the glucose concentration constant, and control by keeping a low constant oxygen level in the medium. For the feeding control based on glucose concentration, a recently developed rapid glucose controlling system was tested for the first time in bacterial cultivations and used to establish the fermentative production of extracellular phytase with E. coli. High activity levels (120 U ml(-1)) at short cultivation times (14 h) were obtained. Even higher activity levels - albeit at longer cultivation times - were reached by applying a feeding control, the main characteristic of which was a constant low oxygen concentration. The optimum oxygen level for the production of phytase was in the range of 5-10% saturation.