1. An ecological survey of the projected area, a tidal flat of 4 km square in Song-Do, Inchon was undertaken from December 1967 to February 1970. 2. Bullacta exarata, Unbonium thomasi, Macrophthalmus dilatatus, M. japonicus, Philyra pisum, Hinia festiva and Batillaria cumingii are the main epifauna; Mactra veneriformis, Meretrix lusoria, Cyclina sinensis, Phacosoma japonica, Moerella iridescens, Solen strictus, Upogebia major and polychaeta are the main infauna in the projected area. 3. The total animal biomass, expressed as wet weight, was an estimated 1,800t in the projected area, of which about 90 per cent was contributed by molluscs. 4. The biomass of commercial clams, Meretrix lusoria, Mactra veneriformis, Cyclina sinensis and Phacosoma japonica was over 65 per cent of the total biomass. 5. The middle tidal zone (3∼5th zones) exhibited high biomass, while the high tidal zone (1∼2) and the low tidal zone (6∼8) exhibited much lower biomass. 6. The author realized that the biomass of primary consumers was approximately 1,700t, and that of secondary consumers was less than 100t. 7. The ecosystem in the projected area is affected very considerably by man.