As nitrogen and phosphorus sources, the treated sewage effluent and the bay-bottom mud promote the eutrophication of estuarial and coastal areas, especially closed bays. The present paper deals with the effects of the treated sewage effluent and the bay-mud exudate on the growth of various phytoplankton species present in natural sea water. 1. Five sampling stations were set in the area receiving the effluent from Fukuoka Eastern Sewage Disposal Plant. 2. Throughout six times investigations from May to November in 1979, the major Phytoplankton occurring at the sampling stations were nine species of Bacillariophyceae and one of Dinophyceae. 3. In summer, the surface water in eastern Hakata Bay was extremely both N and P or N deficient, especially at the bloom of phytoplankton. 4. The average concentrations of NH_4-N, NO_3-N, NO_2-N, org. N, PO_4-P and org. P of the treated sewage effluents used for the N, P enrichment of media were 19.2, 3.71, 0.1, 0.94, 2.64 and 0.20 mg/l, and those of bay-mud exudates were 20.8, tr., tr., 1.47, 3.35 and 0.49 mg/l, respectively. 5. In the N, P enriched media with NP-water (NO_3-N, 16 mg/l; PO_4-P, 2 mg/l), the cell number of almost all phytoplankton species seeded with natural sea water increased with increasing N, P concentrations of the media. In the media enriched with the treated sewage effluent and the bay-mud exudate, however, the growth of most phytoplankton species except Skeletonema costatum and Thalassiosira sp. were inhibited with increasing concentrations of the sewage effluent and the bay-mud exudate in the media. 6. The difference of susceptibility among phytoplankton species to some growth inhibitors which are contained in the sewage effluent and released from the bay-bottom mud may be a factor causing some dominant species in the plankton community.