A quantitative survey of the polychaetes was made at 28-day intervals from floating docks in Los Angeles Harbor for a 17-month period. Additional data consisted of water temperature, turbidity, chlorinity, dissolved oxygen, phosphates and nitrates — nitrites. Stations were selected according to variations in the degree of pollution. Each station was characterized by at least one unique polychaete. The number of species present at a station decreased as the degree of pollution increased. Capitella capilata comprised an increasing percentage of the total polychaete population from the outer harbor to the inner harbor. Polychaetes were absent from the highly polluted station in the inner harbor. The relationship of the chemical and physical data to the polychaete populations was discussed.