Thermoactinomyces vulgaris R-47 produces two a-amylases, TVA I and TVA II, and both enzymes catalyze the same transglycosylation reaction. However, TVA II accumulates the transglycosylation products more than TVA I. The time course of the accumulations between the two TVAs was compared using pullulan and [U-→¹⁴C]glucose. The reactions consisted of two stages, which are the formation of the transglycosylation products and the degradation of the accumulated products, whether TVA I or TVA II was used. The rates of product formation by TVA I were similar, 1 to 2.4-fold, to those by TVA II, whereas TVA II accumulated 3.5 to 7.5 times as much products as TVA I. Thus, we infer that the rates of product degradation by TVA I were larger than those by TVA II. The difference in the degradation rates was not due to the hydrolytic activity, ie., differences in the kcal/Km values between TVA I and TVA II, but due to the inhibition of TVA II by panose, the hydrolysate of pullulan.