In the previous papers, the authors have reported the distribution of nitrogen fixing bacteria in natural water environments. This paper concerns the occurrence of the bacteria in fish culture ponds, in which the concentration of nutritional salts is extremely high, in addition to the large population of total heterotrophic bacteria, i.e. 104-106 cells/mL in the water and 106-107 cells/g in the bottom sediments.
Even in such eutrophic water regions, the standing crop of nitrogen fixing bacteria was not larger than that in the other water environments, namely 0.1-10 cells in 1mL of the water. A diurnal change in the number of the bacteria was not observed in spite of the considerable fluctuation of the physical environment in these ponds. In the bottom sedi-ments of the culture ponds, the bacterial standing crop was 102-103 cells/g, which was com-parable with that in natural eutrophic ponds.