During toxic Alexandrium tamarense blooms in the area of the Orkney Islands and the Firth of Forth (Scotland) in 1998, two drifting experiments were performed and water samples were taken for molecular analysis of bacterial populations associated with the blooms. Genomic bacterial 16S-rDNA was amplified by PCR from three sample fractions (< 3 µm > 0,2 µm, < 10 µm > 3 µm & < 100 µm > 10 µm). Bacterial community structure determined by DGGE (Denaturing Gradient Gel Electrophoresis) showed differences between the sample fractions but only slight variation during each drifting experiment. From two stations of one drifting experiment bacterial 16S-rDNA of the largest sample fraction (< 100 µm >10 µm, mostly dinoflagellate cells) was cloned and a number of 39 clones were sequenced. Additional, from the same samples bacteria were isolated on low nutrient media and the 16S-rDNA was also sequenced. >From 39 clones of the largest sample fraction, 25 clones were assigned to the a-subdivision of proteobacteria. Cultivation experiments on low nutrient media lead to the isolation of fast growing g-subdivision proteobacteria. In contrast to other studies, the particulate sample fraction was dominated by a -subdivision proteobacteria. This sample fraction consisted of mostly viable dinoflagellate cells. Hence, our results indicate, that a -subdivison proteobacteria are closely associated to the blooms of toxic Alexandrium tamarense.