Interactions between bacteria and species of harmful and/or toxic algae are potentially important factors affecting both the population dynamics and toxicity of these algae. Recent reports of bacteria lethal to certain harmful algal bloom (HAB) species, coupled with a rapidly evolving interest in attempting to minimize the adverse effects of HABs through various prevention, control, and mitigation strategies, has focused attention on defining the role of algicidal bacteria in bloom termination. The aim of the present study was to determine if algicidal bacteria active against Karenia brevis, a dinoflagellate responsible for frequent and protracted red tides in the Gulf of Mexico, are present in the waters of the west Florida shelf. To date we have detected one algicidal bacterium from samples obtained during ECOHAB-Florida cruises and we are continuing this effort. A taxon-specific 16S rRNA probe was developed for a previously isolated algicidal strain, 41-DBG2, and we have begun to screen water samples for its presence using fluorescent in-situ hybridization. In addition, we have employed denaturing gradient gel electrophoresis (DGGE) to characterize the diversity of the ambient microbial assemblages and to visualize the band characteristic of strain 41-DBG2. This band can then be analyzed further by excising and sequencing. Our screening results to date have yielded negative results on all but one of the field samples tested, which was positive with the rRNA probe. These largely negative findings may not be unexpected due to the marked temporal and spatial separation of the original algicidal bacterial isolation and the water samples being screened. Future efforts will focus on isolating algicidal bacteria from the same bloom event as the samples for screening are obtained, thereby increasing the likelihood of detecting algicidal strains and permitting a more effective analysis of their population dynamics.