In addition,n itratew asn ot accumulateind the water when the alginate gel beads were packed, whereas the opposite was observed in the control experiment. This indicates that denitrification is active in the alginate gel beads as has been noted previously (7). Around the gel beads, organic matter such as feces of carp and microorganisms in the water was accumulated like sediment mud. This organic matter was decomposed to volatile fatty acids or other organic acids by digestion under microaerobic conditions. Strain S probably utilized these organic acids as substrates (electron donor). Gel beads, however, began to decompose after 2 weeks and COD removal activity began to decrease.