The aim of the present study was to detect the levels of NH⁺₄-N,NO^-₂-N and NO^-₃-N,and to determine the numbers of ammonifying,nitrifying,and denitrifying bacteria,which are responsible for the nitrogen cycling process in four representative grass carp(Ctenopharyngodon idellus)ponds.After sampling water from 4 grass carp ponds,one hundred strains selected randomly from each sample were tested by qualitative color reaction,and on the basis of the test,11 strains of heterotrophic bacteria were selected for 16S rRNA sequence analysis.Results showed that the mean levels of NH⁺₄-N and NO^-₂-N was 5.597 mg/L and 0.135 mg/L,respectively.The cultivable heterotrophic bacteria density in fish pond was 3.26×10⁵ cfu/mL and 89.75% of them were involved in nitrogen cycle,65.75% and 31% of which were involved in NH⁺₄-N production and degrading,respectively.This result indicated that the overwhelming majority of nitrite-producing bacteria resulted in high level of nitrite in grass fish pond.The identification results of 11 strains of heterotrophic bacteria were as follows:six of them were related to Stenotrophomonas,three of them were related to Pseudomonas,and the other two strains were Klebsiella and Enterobacter,respectively.Moreover,there might be strain differences on utilization of nitrogen resourses.