Population densities of Pythium fluminum Park var. fluminum (P. fluminum) in pond water are given for a year sampling period. Colony number per liter ranged from 0 to 260. A fluctuation in population density is clearly detectable being highest in winter and decreasing gradually until reached zero in summer. When the pond water temperature exceeded 25℃ the fungus was not detected. P. fluminum is different from any other species of Pythium in its capability for decomposing insoluble cellulose and reducing the filter paper weight by 20% in one month. A quantitative estimation of this fungus can be carried out by filtering pond water through filter paper circles using suction force. The filter papers were then seeded on CPV selective medium. After keeping for two weeks at 15℃, the pale salmon pink colonies could easily be counted
