Currently, the knowledge of ecology and function of bacteria harbored by molluscs is very limited. In this study, quantity and richness of culturable heterotrophic bacteria (CHB) in seawater of the farming environment and guts of the oyster Crassostrea hongkongensis were determined monthly, with classical, biochemical, and molecular methods. About 800 randomly selected bacterial strains were purified and identified.Quantities of CHB and vibrios, ranging from2.3×10³ to 2.2×10⁵ CFU/ml and 1.6×10² to 3.0×10³ CFU/ml, respectively, were present in the farming seawater, whereas those in the oyster C. hongkongensis ranged from 5.3×10⁴ to 5.4×10⁶ CFU/g and 6.0×10³ to 1.4×10⁵ CFU/g, respectively. Vibrio spp., Pseudoalteromonas spp., Bacillus spp., and Shewanella spp. appeared almost every month. Richness of heterotrophic microbial communities of the seawater and the oysters was very similar and peaked in August. Variation in the diversity (Shannon index) of CHB in the seawater and in the oysters was similar, a significant positive correlation was showed between them (r = 0.894), and their diversities were both peaked in August. Statistical analysis showed that salinity hasmore important impact on quantity of CHB in seawater. Similarly, temperature hadmore important impact on that in the oysters. Important threshold data were analyzed to further explain the relationships.