||The ubiquitous bacterioplankton are major components of food webs and thus play key roles in biogeochemical cycles and energy flow in marine ecosystems. The subtropical Hong Kong waters, which consist of contrasting coastal environments (meso-trophic, polluted, and eutrophic), were good model ecosystems for microbial studies. This thesis investigated the diversity and spatio-temporal dynamics of bacterioplankton from 2004 to 2006 in Hong Kong waters. The bacterial mortality by flagellates and viruses were evaluated in the laboratory. Bacterial abundance was higher in summer than in the winter season in Hong Kong waters. The major bacterial phylogenetic affiliations were Proteobacteria, Bacteroidetes and Cyanobacteria, revealed by 584 16S rRNA gene sequences from DGGE and clone library analysis. Spatially, bacterial communities in three stations of Victoria Harbor were similar, but differed from those in adjacent coastal and estuarine stations. Particle-attached and free-living bacterial community composition differed in the Victoria Harbor area. Abundant Roseobacter sp., Legionella spp., Arcobacter sp., and Glaciecola sp. were observed in Hong Kong waters. Clear temporal patterns were shown for bacterial community structure from 2004 to 2006, except for Victoria Harbor West (VHW) close to the sewage discharge outfall. Temporal variations in bacterial community structure were best related to temperature at most stations except at VHW, which was best explained by pollution indicator parameters (e.g. biochemical oxygen demand) by using BIOENV analysis. Temporal patterns of bacterial diversity index, species richness, and community structure, as well as relative abundance of major bacterial lineages (e.g. Roseobacter sp., Synechococcus sp., Cytophaga sp., Legionella sp., etc.), indicated the combined effects of hydrological conditions and anthropogenic pollution on bacterioplankton in Hong Kong waters. Flagellates and viruses affected bacterioplankton in Hong Kong waters as well. Significant repressing effects on bacterial abundance and production were observed in laboratory experiments for in situ bacterial assenmblages. Cluster analysis of the DGGE patterns showed that the effects of viruses and flagellates on bacterial community structure were relatively stochastic, while the co-effects of predators caused consistent trends and substantially increased the apparent richness. This first systematic attempt to study the interactive effects of viruses and flagellates on the diversity and production of bacterial communities in coastal waters suggests that tight control of bacterioplankton dominants results in relatively stable bacterioplankton communities. During my study, different molecular techniques (T-RFLP, DGGE, and clone library) were compared and evaluated for their application in marine bacterioplankton. The biases associated with the choice of digestion enzymes shall be carefully evaluated, or at least addressed when using T-RFLP analysis. The detailed information of bacterial diversity, dynamics, and mortality substantially improved our understanding of the roles of bacterioplankton in this subtropical coastal marine environment.