Investigations into eutrophication of Ngau Mei Hoi Bay, Hong Kong, were carried out in July and December 1996. The results show that Ngau Mei Hoi Bay can be classified as hypereutrophic. During summer phytoplankton blooms (average chlorophyll a = 33.96 μg L−1), high oxygen saturation (up to 145.3%), high pH (7.8–8.4), and nutrient depletion (mean NO3 NO2 = 0.89 μg L−1) at the surface in association with hypoxic (dissolved oxygen concentration was down to 0.9 mg L−1), low pH (down to 4.9), and high nutrient (mean NO3 NO2 = 96.28 μg L−1) conditions at the bottom layer were observed. This typical phenomenon of eutrophication resulted from nutrient enrichment by anthropogenic activities, water column stratification formed by spreading of the Pearl River plume at the surface, and outer shelf water intrusion at the bottom. The perniciousness of the hypereutrophication and future monitoring needs for environmental management are also discussed.