The effects of chemical composition of alginate (i.e., M/G ratio) and gelling agents on the effective diffusivity DE Of oxygen, nitrate and glucose were investigated for preparation of an immobilization matrix to entrap denitrifier. The ratio of D-E of oxygen to D-W in water for Ba-alginate gel was the lowest among Ca-, Sr- and Ba-alginate gels, and the value was only about one half. With increasing M/G ratios above about 0.5, the value of D-E of nitrate or glucose of alginate gels tended to decrease. Ba-alginate gel with M/G ratios below 0.5 was found to be suitable for immobilization of denitrifier. The effects of bulk dissolved oxygen (DO) concentration on the denitrification activity of the immobilized denitrifier was investigated. At DO concentrations up to about 2.5 mg/l, the immobilized denitrifier in Ba-alginate gel had almost the same activity to that under anoxic conditions. In addition, denitrification activity under oxic conditions was highest at a cell loading of about 3 g/l, and increased with increasing bead radius.