A growing body of evidence suggests that algicidal bacteria may play an important role in naturally regulating the development and termination of harmful algal blooms (HABs). Interest in such microbes has been enhanced further by their potential use as part of a HAB management strategy. We have isolated two bacterial strains from the west Florida shelf that are lethal to Gymnodinium breve, a bloom-forming dinoflagellate responsible for severe economic losses in this region through impacts on fisheries and tourism. A previous report on these bacteria described aspects of their killing activity and the taxonomic specificity of target organisms. We have now completed sequencing of the 16S rRNA gene for both algicidal bacteria and preliminary analyses suggest that one strain resides in the flexibacter-cytophaga subgroup of the cytophaga/flexibacter/bacteroides (CFB) phylum, while the other strain is a member of the gamma-proteobacteria. Fluorescently-labeled rRNA probes have been developed for both taxa and are being employed via in situ hybridizations on both laboratory and field samples to begin characterizing the population dynamics and distribution of these algicidal strains. Progress is also being made toward the identification of the algicidal compound(s) produced by one of the bacteria. A high-throughput bioassay for guiding fractionation of extracellular bacterial metabolites based on algicidal activity has been developed. Using this approach, chromatographic separation of an algicidal component from bacterial culture filtrates has been achieved, and further purification as well as structural analyses using mass spectrometry and NMR are underway. We will present a summary of laboratory and field results to date describing these G. breve killing bacteria and their algicides. Finally, a consideration of all algicidal bacteria reported thus far from different geographical regions will provide evidence for the ubiquitous nature and close phylogenetic affinities of these microbes, and thus their potential importance as regulators of HAB dynamics.