1. I observed the seasonal variation in species and quantity of plankton appeared in 3 irrigation ponds, O-ike, Sin-ike and Asama-ike, from March to September 1947.
2. The numbers of species of phytoplankton and zooplankton are almost equal: about 67 species in O-ike, 60 in Sin-ike, 45 in Asama-ike.
3. Productivity of plankton is greatest in Sin-ike, and smallest in Asama-ike. The largest number observed was 4828/l in June in Sin-ile, 832/l in September in O-ike, 795/l in April in Asama-ike,
4. Phytoplankton appeared much in Spring and Autumnin Sin-ike and O-ike, but in Asama-ike in Spring only.