There is increasing evidence that bacterial-algal interactions play a role in Harmful Algal Bloom (HAB) ecology. Bacteria have been implicated in the production of paralytic shellfish poison (PSP) toxins, which are normally associated with bloom forming algal species, specifically toxic dinoflagellate algae. To clarify the role that these bacteria may play in the production of PSP toxins, it is desirable to identify and localise the toxic bacteria associated with the dinoflagellates and the toxic algal blooms that they produce. 16S rRNA-targeted probes offer the possibility for both and thus, probes have been made to putatively toxigenic bacteria isolated from the PSP-related dinoflagellate Alexandrium tamarense and tested for their specificity in dot blot and in-situ hybridisation experiments using cultured isolates. The bacteria belong primarily to the a-Protoebacterial group of Roseobacteria and the g-Proteobacterial group of Alteromonas. Results are also presented from field tests of the probes as well as localisation of these bacteria in cultures of the dinoflagellates using confocal microscopy. The major abundance of the bacteria recognised by the probes occurred at two sites around the Orkney Islands and preceded the major Alexandrium tamarense bloom, which also occurred at the same two sites, by two weeks.