Cyclodextrins (CDs) are produced industrially from starch using bacterial cyclodextrin glucanotransferases (CGTase, EC 2.4.1.19). Batch cultivation of alkalophilic Bacillus circulans ATCC 21783 for CGTase synthesis was performed in a fluidized bed reactor. Under optimal growth conditions (40 degreesC, 150 ml working volume, 3.3 v/v/m, 1.0-2.0 g l(-1) initial starch concentration) enzyme activity was in a range 120-150 U ml(-1) at a specific growth rate 0.55-0.60 h(-1) (generation time 75-80 min). CGTase was produced intensively after the end of the exponential phase (from 14th to 30th h) and thereafter the enzyme yield remained in the denoted range for 48 h. The use of agar-entrapped cells of B. circulans ATCC 21783 for CGTase production in a fluidized bed reactor led to enzyme activity in the range 180-210 U ml(-1) after 24 and 48 h cultivation, respectively. The operational stability of the biocatalysts was studied by repeated batch cultivation for 240 h (in a reactor). The residual activities represented 90-95% of maximal. Scanning electron microscopy observations showed large groups of vegetative cells which continued to grow rapidly inside the agar beads indicating that high CGTase activity was due to the immobilization of the cells. (C) 2003 Elsevier Science Ltd. All rights reserved.