The dinoflagellate Karenia brevis is capable of significant ecological and economic impacts in Florida waters where blooms typically occur. Blooms and cultures of K. brevis were sampled to determine the composition, production, and possible ecological function of bacteria and virus communities associated with K. brevis. Bacterial communities on the West Florida Shelf (WFS) were similar inside and outside K. brevis blooms, but primary and secondary (bacterial) production and bacteria and virus abundances were different depending on bloom stage. Bloom stages need to be identified so that discrete sampling events can be combined to characterize an entire bloom event. Within an initiating bloom bacterial production and mortality was high and viral abundance was low, suggesting that viral genomes were either within host cells or bacterial mortality was due to mixotrophic grazing by K. brevis or heterotrophic nanoflagellates. In a maintenance phase bloom the bacterial community was metabolically stressed, subject to increased viral infection, and most likely not being subjected to mixotrophic grazing. Bacterial communities associated with healthy K. brevis were dominated by the Cytophaga-Flavobacterium-Bacteroides (CFB) complex. As K. brevis shifted to stationary or senescing growth communities had higher proportions of Alphaproteobacteria. The SAR406 group, typically found in deep waters, was present in the surface waters of the WFS which supports existing K. brevis bloom formation hypotheses involving upwelling of deep waters from the mid to outer shelf. The CFB complex of bacteria also need to be further investigated as the consistent presence of CFB bacteria in both blooms and cultures of K. brevis suggest CFB bacteria are capable of numerous interactions with K. brevis. Furthermore, such interactions may be a vector of bloom control through viral infection; a high proportion of CFB bacteria would be ideal for density-dependent viral infection which could disrupt interactions between bacteria and K. brevis. Inoculating cultures of K. brevis, which included associated bacteria, with viral concentrates from the WFS showed differences in bacterial production and growth which indicate viruses are acting upon the bacterial community and not the dinoflagellates. Interactions between bacteria and K. brevis need to be further elucidated and explored for a better understanding of the role of each in dynamics of this harmful algal species. There may be a natural community succession amongst bacteria during blooms: utilizing certain indicator species to indicate bloom stage and transition between stages may aid in bloom forecasting and detection efforts.