Spores of Aspergillus niger 26 were immobilised in calcium alginate and the immobilised growing mycelium reused in batch mode for the production of polymethylgalacturonase (PMG). The influence of alginate concentration, initial inoculum gel volume, spore concentration, and duration of the fermentation cycle on enzyme activity and mycelium growth are reported. The best yield of PMG was reached with 5 mi initial gel volume 3% alginate concentration, 10(9) spores/ml gel and a cycle time of 72 h. The optimisation of these factors, which affect the stability and productivity of the immobilised system, resulted in an increase in enzyme activity and the possibility of semicontinuous cultivation.