In the case of the fish-culture-pond we have hitherto no appropriate direct method to measure and denote the amount of phytoplankton which plays an important role upon the productivity of the pond. Although the fact that the close relation exists between the water transparency measured by the Sccchi disk and the amount of seston is well known, such a large disk as the Secchi is not available in the shallow water of ordinary fish ponds. The author deviced a practical, convenient method for the prerent purpose. Using a white china disk, 5cm in diameter, the transparency is measured by dipping it into the water and reading the depth where it becomes invisible. A Relationship represented by the hyperbolic curve exists between the observed transparency and the precipitation volume of the phytoplankton. Some transparency measurements in ponds at Ikawazu, Aidprefecture are shown.