A nitrifing bacterial strain Nitrosomonas europaea ATCC 25978 was succesfully immobilized onto calcium alginate, which was selected as a suitable carrier among many other material tested. The optimal conditions for immobilization were: Na-alginate, 4% (w/v); concentration of CaCl_2, 0.1 M; immobilizing time, 20 min; cell concentration 60 μg/ml, diameter of alginate bead, 5 mm; precultivation time of cells for immobilization, 24-48 hr. Optimal cultivations for the immobilized cells were: ammonium sulfate concentration, 1 g/l; initial pH, 8.0; temperature, 30°C. One mM Mo^6+ and 0.2% C. S. L. accerelated the growth 65 and 85%, respectively. Continuous cultivation was performed for 13 days.