Abstract: A strain which can effectively degrade NH₃-N in the fishery water was obtained from sea water by culturing in the isolating medium in which the concentration of NH₃-N was 212 mg/L. It was identified as Candida sp. The factors affecting the degradation of NH₃-N by Candida sp.such as the concentration of NH₃-N, pH value, temperature were studied. The results showed that the suitable conditions were as follows: the concentration of NH₃-N≤20 mg/L, pH value 6～7, temperature 25～30℃, the salinity 0～1% and the dissolved oxygen concentration >2 mg/L. under this condition, the degradation rate of NH₃-N can reach 80% when the inoculum of Candida sp. was 5%.