An unconventional immobilization of dextransucrase via entrapment in alginate is described, which provides high activity yield and good operational stability. This method is normally restricted to the immobilization of complete cells or parts of cells. Experiments were designed to provide evidence for the immobilization mechanism. It was shown that a typical globular protein, alpha-chymotrypsin, is neither immobilized in an alginate matrix nor by formation of an additional network of dextran inside alginate. The results suggest that dextransucrase immobilization in alginate is due to a unique supramolecular structure.