Chemical modification of glucoamylase [EC 3.2.1.3] from Rhizopus niveus by N-bromosuccinimide was carried out to investigate the role of tryptophan residues in the enzyme-catalyzed reaction and their location in the enzyme subsites. Of the ten tryptophan residues of the enzyme four could be modified. The two more reactive residues were confirmed not be essential for the catalytic activity for the hydrolysis of maltodextrin and phenyl alpha-maltoside. Complete loss of the catalytic activity, however, was brought about by modifying the two less reactive residues, and the modification of these residues was prevented by the substrates. The characteristic difference spectrum produced by maltose (7) disappeared in parallel with the loss of the catalytic activity. These results suggest that the tryptophan residue(s) responsible for the maltose-induced difference spectrum may be located at one of the subsite near the catalytic site and plays an important role in the catalytic activity of the enzyme.