An exo-beta-1,4-galactanase was previously purified from Aspergillus niger. It catalyzed the hydrolysis of potato galactan and several galactooligosaccharides to produce galactose. In the present work, HPLC analysis of the reaction products showed intermediate oligomers with higher degrees of polymerization (dp) than initial oligomers. The exogalactanase was shown to transfer galactose residues to galactooligosaccharides. The influence of several parameters has been studied, such as the nature and concentration of donor (galactan or galactose), the effect of pH and temperature, and the amount of enzyme. Transgalactosylation, which was studied as the yield in reformed oligomers, was found to be most efficient when acceptor was used at a ID mM final concentration and galactan (1.5% w/v) was donor with 5 nkat ml(-1) exogalactanase at pH 7 and 40 degrees C. When the dimer was incubated under these conditions, the yield in reformed oligomers was 30.9%. Transfer reactions were analyzed using oligomer with dp 2-5 as acceptor Each incubation parameter was shown to influence the transglycosylation in the same way despite the dp of the initial oligomer. Transfer products were move stable at pH 7 than at pH 3.5 but were finally rehydrolyzed in galactose. From experimental results,the enzymatic mechanism involving hydrolysis and transglycosylation is proposed.