After a great mortality owing to abnormal oceanographical condition and fungal disease in 1973 hanging cultch lines per raft has been reduced to 450 from 558, and oyster production per raft has also been decreased. It seems to be result of dense culture of oysters and its resulting accumulation of waste materials on the sea bottom in the farm area. The present study was carried out to investigate the effect of rearing density and the degree of eutrophication in Geoje Bay for 6 months from June through November in 1977. Total area of this bay is about $48.9;km^2$, and the area of registered oyster farms as of November in 1977 is around $10.9;km^2$, which is about $22.3\%$ of this bay. Water Quality during summer season was shown as transparency 5.5 m, COD 1.5ppm, degree of oxygen saturation $90.6\%$ in upper and middle layers and $82.2\%$ near bottom, chlorophyll-a 3.0 mg/m^3, and phytoplankton $8.7 imes10^4;cells/l$. In superficial mud in August COD was 35.4 mg/g, total sulphide 0.24 mg/g, and phaeophytin $43.7;{mu}g/g$dry mud. These values indicate that water qualify in this bay is so far excellent but quantities of chemical oxygen demend and sulphide in bottom mud show maximum level or a little over eutrophication standard.