Physiological condition of phytoplankton can be estimated from both quantitative and qualitative analyses of photosynthetic pigments. In this study, the process of chlorophyll a degradation in an enclosed eutrophic bay were investigated using high-performance liquid chromatography (HPLC). Chlorophyll a and its derivatives were classified into chi. a, chl. a isomers (chi. a'-1, 2, 3) and pheopigments by the use of reverse-phased HPLC. Resulted pigment compositions were compared between suspended particles, sinking particles and surface sediment in Etauchi Bay. Chl. a was more abundant in suspended particles than in sinking particles, while pheopigments in both particles accounted for only less than 30%. Since it was reported that 66% of chl. a was degraded by grazing of zooplankton, these results indicate that grazing and digestion by zooplankton are not the major process of degradation. Different from open sea, the major degradation process of chl. a in the eutrophic coastal area was concluded to be the natural death of phytoplankton. Because of the shallowness of the area, final stage of the degradation is supposed to occur after sedimentation.