The microorganism Talaromyces duponti ATCC 20805 can produce an extracellular glucosyltransferase (EC 2.4.1.24) with a high transglucosylating activity. A 100-h culture using maltodextrins as the carbon source gave an enzyme production of 11 U ml(-1). The enzyme was further purified to homogeneity by fractionation steps involving ammonium sulfate precipitation, hydrophobic interaction chromatography, ion-exchange chromatography, and chromatofocusing. The enzyme gave a single protein band on gel electrophoresis (Mr = 170,000) bus two protein bands on isoelectrofocusing (pl values 4.19 and 4.26). It is composed of two isoforms both showing transglucosylase activity. The two isoforms could be separated and their carbohydrate content evaluated These were equal to 2.30 and 2.84% (w/w). The optimum pH and temperature for the enzymatic reaction are 4.5 and 70 degrees C, respectively. The transglucosidase retained 50% activity after 73.2 it at 60 degrees C. The high thermostability of the enzyme makes it particularly suitable for alpha-butylglucoside synthesis in biphasic medium by transfer of glucosyl moieties from maltooligosaccharide donors onto butanol.