This paper summarizes the results of four years of investigation on the ecological effects of sewage discharge and its pollutants such as organic loading, petroleum hydrocarbons, chlorinated hydrocarbons and toxic metals on marine organisms and fishery resources in Bohai Bay. The results showed that the structure and function of organism communities in the bay were essentially stable. No great difference in the species composition and the avarage quantities of various population (Microbe, Phytoplankton, Zooplankton, Fish eggs and Juneniles, Benthos) from that in 1958-1959 were found. However, ecological modification has appeared in some areas contaminated by the municipal and industrial sewage from the Tianjin city and towns near the seacoast. The changes including the increase of concentrations of metals, DDT and 666 in organisms and the decrease in diversity values and abundance of some populations seem mainly limited to the estuarine area, where rather high mortality appeared occasionally of mollusca, shellfish as well as fish. Catch statistics of commercially valuable species in estuarine waters reveal that changes in abundance are directly attibutable to pollution. Results show that the estuary was affected by pollution.