The enzyme cyclodextrin glucosyltransferase (CGTase), EC 2.4.1.19, which produces cyclodextrins (CDs) from starch, was obtained from Bacillus firmus strain no. 37 isolated from Brazilian soil and characterized in the soluble form using as substrate 100 g/L of maltodextrin in 0.05 M Tris-HCl buffer, 5 mM CaCl2, and appropriate buffers. Enzymatic activity and its activation energy were determined as a function of temperature and pH. The activation energy for the production of beta- and gamma -CD was 7.5 and 9.9 kcal/mol, respectively. The energy of deactivation was 39 kcal/mol. The enzyme showed little thermal deactivation in the temperature range of 35-60 degreesC, and Arrhenius-type equations were obtained for calculating the activity, deactivation , and half-life as a function of temperature. The molecular weight of the enzyme was determined by sodium dodecyl sulfate poly acrylamide gel electrophoresis, giving 77.6 kDa. Results for CGTase activity as a function of temperature gave maximal activity for the production of beta -CD at 65 degreesC, ph 6.0, and 71.5 nmol of beta -CD/(min mg of protein), whereas for gamma -CD it T-vas 9.1 mmol of gamma -CD/(min mg of protein) at 70 degreesC and. pH 8.0. For long contact times, the best use of the enzymatic activity occurs at 60 degreesC or at a lower temperature, and the reaction pH map be selected to increase the yield of a desired CD.