During the purification of an aquarium for carp breeding, a relatively high level of chemical oxygen demand (COD) was removed by filtration systems packed with both alginate- and polyvinyl alcohol (PVA)-immobilized gel beads of Rhodobacter sphaeroides S. Low nitrate accumulation was observed in the alginate gel beads packed system due to denitrification, but high levels of nitrate and nitrite accumulation were observed in the PVA gel beads packed system. This phenomenon was caused by the inhibitory effect of PVA on nitrite reductase. The boric acid used for hardening gel beads of PVA slightly inhibited nitrate reductase. On the other hand, during the denitrifying growth experiments for this strain, boric acid inhibited cell growth, but PVA only partially inhibited cell growth. Based on electron equivalent (Y(eq.)), growth yields using various kinds of concentrations of PVA were almost identical. It was suggested that PVA might only limit the growth rate of this strain by the inhibition of nitrite reductase.