Gymnodinium catenatumis an introduced toxic dinoflagellate that blooms intermittently and causes shellfish farm closure in the Huon Estuary, Tasmania, Australia. Seventy-five bacteria isolated from the estuary were tested for algicidal activity against this and other toxic and non-toxic algal species. Five isolates produced algicidal extracellular exudates. These algicidal species were aPseudoalteromonassp. (ACEM 4), a novelZobelliasp. (ACEM 20), a strain ofCellulophaga lytica(ACEM 21) and 2 Firmicutes: a novelPlanomicrobiumsp. (ACEM 22) and a strain ofBacillus cereus(ACEM 32). This study is the first time Gram-positive bacteria have been associated with algicidal activities. Further data are presented on an algicidalPseudoalteromonasspecies previously isolated from the Huon Estuary (Strain y). Supernatant produced by all 5 strains caused cell lysis and death inG. catenatumvegetative cells. No change or reversible ecdysis was noted for 2 other endemic dinoflagellate species. Algicidal or inhibitory activity was not activated via homoserine lactones, but bacterial quorum sensing for the isolates was shown by means of the AI-2 mechanism. Algicidal activity from field isolates was also influenced by strain or environmental variation. Bacteria were capable of losing or switching off their algicidal ability indicating that the presence of an algicidal species in the environment may not necessarily signify that they are currently algicidal. Concentrations of algicidal compounds required for algal lysis in laboratory experiments indicate that the 5 bacterial species can be effective againstG. catenatumvegetative cells if they dominate the bacterial population in the estuary, particularly when attached to particles.