Different alginate systems, viz., sodium alginate, glutaraldehyde-treated alginate and PVA-alginate gel were examined for the immobilization of whole cells of Aspergillus niger. The enzyme activity of sodium alginate immobilized cells was greater as compared to the other two systems. In the process, dried orange peel powder served as a very good inducer of PGase. Maximum enzyme production was observed with 3% alginate and 1.5% orange peel inducer in 72 h the ratio of gel beads/medium (v/v) being 1:4. The immobilized beads could be effectively reused three times with a loss of 5% activity. After the third cycle, there was a loss of 3% with each consecutive use till the sixth cycle. The present immobilization system indicates the possibility of its application in a semi-continuous process for the production of polygalacturonase. (c) 2005 Elsevier Ltd. All rights reserved.