The action of cyclodextrin glucanotransferase (CGTase, EC 2.4.1.19) on amylose was reinvesti-gated by determining the product specificity of the cyclization reaction and the substrate specificity of the coupling and hydrolytic reactions of CGTases from alkalophilic Bacillus sp. A2-5a (A2-5a CGTase), Bacillus macerans CGTase (Bmac CGTase) and Bacillus stearothermophilus CGTase (Bste CGTase). The product specificity of the cyclization reaction of CGTase was determined by quantifying each cyclic α-1, 4-glucans with degree of polymerization (DP) from 6 to 31 (CD6 to CD31; CDn, cyclic α-1, 4-glucan with DP of n) produced at the initial reaction stage with HPAEC. All three CGTases produced CD6 to CD31 simultaneously. However, A2-5a CGTase produced more CD7, while Bmac CGTase produced more CD6 than other cyclic α-1, 4-glucans. The speci-ficity to produce CD7 or CD6 was very strict since the cyclic α-1, 4-glucan having a difference of only one glucose unit were not preferably produced by either enzyme. 4n the other hand, Bste CGTase produced more CD6 and CD7 than other cyclic α-1, 4-glucans, respectively. The substrate specificity of the coupling and hydrolytic reactions were determined by using CD6, CD7, CD8 and larger cyclic α-1, 4-glucans (mixture of CD22 to CD50). For the coupling and hydrolytic activities of all three CGTases, CD7 and CD8 were extremely poor substrate, while those activities on larger cyclic α-1, 4-glucans were higher than CD6, CD7 and CD8. The substrate specificity in the cou-pling and hydrolytic reaction could be attributed to the conversion of larger cyclic α-1, 4-glucans into smaller ones. The hydrolytic activity of CGTases on larger cyclic α-1, 4-glucans were 1/1.6 to 1/5 of the coupling activity, which indicated that the hydrolytic activity of CGTase was not so small as negligible. The hydrolytic activities of Bste CGTase were the highest among the three CGTases, while those of Bmac CGTase were the lowest, which would cause the difference in the conversion rate from larger cyclic a-1, 4-glucans into smaller ones. Based on these results, we discussed the action of CGTase on amylose.