Bacillus subtilis cells were immobilized by entrapment in calcium alginate gel capsules and the immobilized biocatalyst was used for the semi-continuous production of a-amylase. The alpha-amylase yield and operational stability of the immobilized system were increased by tailoring the capsules' characteristics. Capsules prepared from 2% (w/v) sodium alginate and 3.5% (w/v) CaCl2 were the best support for cell immobilization, providing 2.5-fold higher alpha-amylase production in comparison to the freely suspended cells. Immobilized biocatalysts sustained 90% of their initial productivity over five sequential batches in a 10-day period, while amylase production by free cells declined sharply after the second use. Even higher operational stability was achieved when the capsules were treated with 2% (w/v) CaCl2 for 30 min before each batch. In this case the appearance of free cells in the medium was minimized and the immobilized system retained more than 85% of its initial efficiency after 15 fermentation batches, producing more than 1,450,000 units of extracellular (c)-amylase during this period. (c) d 2005 Elsevier Inc. All rights reserved.