is a common bacterium found in aquatic environment and intestines of warm-blooded animals and human. In summer, it can reach high level in sewage-polluted seawater and be accumulated by scallops. Large numbers of bacteria may constitute significant health risk to cultured animals at high temperature or in other specific conditions E. faecalis in scallop remains unclear, and information about hosts’ growth performance, innate immunity and disease outbreak after a long-term exposure to non-pathogenic fecal bacteria is limited. and variation in immune enzyme activity in the scallopThe findings of this study will be useful for developing aquaculture probiotics and will provide scientific basis for sustainable aquaculture of bivalves.the accumulation and distribution of and the variation in activities of two immunity-related enzymes in . After exposure to seawater inoculum of 5 for two weeks, the uptake of bacteria at day 1, 3, 5, 7 and 14 and their distribution in scallop tissues were detected by plate counting method respectively. The activity of acid phosphatase (ACP) and superoxide dismutase (SOD). The uptake results showed that CFU/g)]were tested in scallop tissues, but at 14 d it decreased to the level of 3 d after treatment. However, the rate of bacterial accumulation (RA) was the highest at 1 d, and then significantly decreased (. It indicated that the bacterial accumulation rate in scallops decreased with exposure time. The top level of , over ten times higher than the average density of total tissues, was observed in digestive tract, mantle lobes and gills, over 100 times lower than the average. SOD and ACP activities in cell-free haemolymph significantly increased to the maximum at 7 d, and then decreased. It can be deduced that there was a positive P<0.05). Soluble factors in serum were more sensitive to bacterial accumulation than those in haemocytes, therefore, the serum may be a chief part of innate immune system of bivalves to digest and kill invading bacteria. haemocytes and soluble factors operate in a coordinated way to provide protection from invading microorganisms,Understanding the immune response in bivalves to invasive infections at cellular and molecular levels offers a theoretical base for exploring molluscan probiotics. Further study on variations in activities and gene expression in haemocytes and the roles of different scallop tissues in immune system is required.