4-alpha -Glucanotransferase from a hyperthermophilic archaeon Pyrococcus kodakaraensis KOD1 showed a broad acceptor specificity to various saccharides in an intermolecular transglycosylation reaction. In particular, the enzyme produced large amounts of transfer products of various accepters such as D-glucose, methyl-alpha -D-glucoside, phenyl-a-D-glucoside, and D-xylose. It is suggested that the requirement for an effective acceptor in the intermolecular transglycosylation reaction catalyzed by this enzyme is the pyranose structure with the same configurations of the free C2-, C3-, and C4-hydroxyl groups as D-glucopyranose, like cyclomaltodextrin glucanotransferase (CGTase). However, the enzyme showed some acceptor specificities unlike those of CGTase. Analysis of the action of 4-alpha -glucanotransferase indicated that the enzyme catalyzes an intramolecular transglycosylation (cyclization) reaction of amylose to produce cyclic alpha -1,4-glucan (cycloamylose). The yield of cycloamylose reached 67%, and the degree of polymerization was found to range from 16 to above 55.