The effects of an anionic surfactant, sodium dodecyl sulfate (SDS), on the kinetic behavior in the hydrolysis of amylose with two bacterial alpha-amylases from Bacillus amyloliquefaciens and B. licheniformis were studied. The catalytic rates of both amylases in the present study showed sigmoidal kinetics with increased concentration of added SDS; the rates were increased in the range below the critical micelle concentration (cmc) and then markedly decreased above the cmc. The catalytic rate of the alpha-amylase from B. amyloliquefaciens was more sensitive to the added surfactant than that of the alpha-amylase from B. licheniformis. The diluted SDS at concentrations below the cmc was responsible for the preferential formation of the enzyme-substrate (E-S) complex, and the hydrolytic catalysis of both enzymes was apparently accelerated. Thermodynamic parameters for the E-S complex formation revealed that the apparent enthalpy and entropy changes of both amylases were increased by the addition of the diluted SDS. It is suggested that the increase in hydrolytic rate by the addition of SDS is due to the larger increase in the entropy changes for the E-S complex formation than in the enthalpy changes.