The growth and nitrogen fixation of Azolla filiculoides cultured in polluted pond water and two types of secondary effluent were investigated. The gradual decreases in the growth and acetylene reduction activity (ARA) of the plants in pond water were due mainly to a phosphorus deficiency. In the secondary effluent, ammonium was an important factor limiting growth and ARA. Azolla plants were screened for tolerance to ammonium ion on the basis of their growth on the medium with or without ammonium. Azolla nilotica was the most sensitive species of all, followed by A. rubra. Azolla pinnata van pinnata was the most tolerant one. Nutrient removal capacities by 10 Azolla strains in batch culture were examined. The highest capacities and the growth were found in May for all Azolla strains. In August and December, Azolla strains showed poor growth and low nutrient removal capacities.