As shown in Fig. 2a, COD and nitrate were reduced when 30g alginate gel beads were added to the denitrifying medium. In particular, the relatively rapid reduction of nitrate was observed and approximately 50 mg/l of nitrite accumulation was observed after 10 d, but this nitrite was reduced again after 20d. This phenomenon was in good agreement with the denitrification of R. sphaeroides S in which nitrate was reduced as follows: NO3+NO2+NO+N20+N2 (11). In general, NO was not observed as an intermediate product of this step in the denitrification of bacteria (12). In addition, N20 could not also be detected in the denitrification by the photosynthetic bacteria, strain S (7, 13). From the results in Fig. 2a, denitrifying activity including cell growth was still active in the alginate gel beads.