Preparations of living Pseudomonas denitrificans cells immobilized in alginate gel were used in the denitrification of water. In the presence of an exogenous carbon source the entrapped microorganisms reduced nitrate and nitrite to gaseous products and to achieve complete reduction, carbon to nitrogen ratios of over two were required. The effects on denitrification of particle size and the number of bacteria in the gel were investigated. Apparent K m values for nitrate and nitrite reduction were calculated for free and immobilized cells. When the immobilized cells were incubated in nutrient media, an increase in reduction rate was observed and this was shown to be caused by the growth of cells within the gel particles. Immobilized P. denitrificans cells retained 75% of their initial nitrate reduction capacity after 21 days of storage at +4~ The operational stability of the alginate-immobilized cells was studied both in batch and in a column which was operated continuously. A column (45 g of alginate-cell fibers in 80 ml) denitrified a high nitrate drinking water (100 mg NO3/1) with a rate of 300 ml of nitrate and nitrite free water/day/g of gel. The half life for nitrate reduction was estimated to be 30 days.