Studies on marine bivalves have demonstrated that suspension-feeding bivalves scan influence the function of ecosystems to a great extent. In dense populations, bivalves can dominate total ecosystem metabolism, nutrient cycling , and grazing of primary producers. Oy ster ( Crassostrea gigas ) is a promising species, which has been widely cultured in the neritic waters in the Bohai Sea and the Yellow Sea. Oyster farming has been bringing not only great economic benefit, but also great burden for the neritic ecosystem due to filtering food and releasing metabolic production. To estimate the effects on neritic eco system of o yster cultivation, the experiments were monthly conducted near Xiao Qingdao Island from March to September 2001. The biodepo sition by Crassostrea gigas and the effects of biodeposition on material flux were measured under natural cultivation conditions. Crassostrea gigas used, with the body sizes of 79. 8- 125. 0 mm shell height and 0. 96- 3. 35 g dry tissue weight, were collected from the oyster farm south of Qingdao . Oyster was div ided into three groups, according to their body size, they were small size ( 80- 95 mm) , middle size ( 95- 110 mm) and big size ( 110- 125 mm) . Each group was arranged for five parallel experiments. The results show that the biodeposition rates was 26. 3- 69. 9 mg.. ind- 1..d- 1, 37. 5- 83. 7 mg.. ind- 1..d- 1 and 44. 1- 103. 7 mg.. ind- 1..d- 1 for small size, middle size and big size of Crassostrea gigas, respectively. The amount of biodepo sit was correlated positively with the shell length and weight, but the per gram body weight biodepo sit negatively. With the water temperature rising form March to September, the chlorophyll a concentration in sea water increased from 1. 6.. 0. 3..g..L- 1 to 7. 3.. 0. 5..g..L- 1 and reached the peak values at 18. 3 .. in June, then went down until September, either the TPM, POM, POC and PON. As a consequence, the biodeposition by each group oyster increased from 26. 3 mg.. ind- 1..d- 1 to 69. 9 mg.. ind- 1..d- 1, 37. 5 mg.. ind- 1..d- 1 to 83. 7 mg.. ind- 1..d- 1 and 44. 1 mg.. ind- 1..d- 1 to 103. 7 mg.. ind- 1..d- 1, then decreased to 49. 0 mg.. ind- 1..d- 1, 76. 9 mg.. ind- 1..d- 1 and 88. 8 mg.. ind- 1..d- 1 in Aug ust or September , respectively . Those demonstrated that water temperature and food concentration were important factors to affect biodeposition of Crassostrea gigas .