In pond Ojaga-ike, Chiba Pref., the author carried out a limnological study of the pond with special emphasis on the ecology of aquatic plants. The study started in November 1977 and finished in October 1980. This paper contains the result concerning the seasonal changes in water quality (Secchi disc transparency, water temperature, dissolved oxygen content, pH, chlorophyll a content, COD of both raw and filtered water and total residue) obtained during the whole investigation period. In addition, the paper describes the result concerning the seasonal and annual changes in surface cover of aquatic plants observed in the present pond.