We characterised the spatial and temporal variation in the bacterioplankton assemblage composition during bloom events of different Alexandrium species (Dinophyceae) in the littoral of the NW Mediterranean Sea by means of catalysed reporter deposition fluorescence in situ hybridisation with oligonucleotide probes (CARD-FISH). We studied several Alexandrium blooms through their seasonal development (at La Fosca beach) or in their spatial variability (in Arenys Harbour and Olbia Bay), and we complemented these observations by describing the composition of the bacterial assemblage associated with cultures of Alexandrium species isolated from the same sites. Our studies on natural bacterioplankton assemblages identified the Bacteroidetes lineage and the Alphaproteobacteria as the dominating components during the studied blooms of Alexandrium. Alphaproteobacteria dominated in the La Fosca and Olbia blooms, while bacteria belonging to the Bacteroidetes were abundant in the development phase of the La Fosca beach bloom and in the winter Arenys bloom. Gammaproteobacteria contributed in low proportions without significant changes through the different bloom phases at La Fosca beach and in Olbia Bay, but were more abundant in Arenys Harbour. While the absolute bacterial abundances in the spatial study of Olbia Bay covaried with the Alexandrium densities, there were no spatial changes in the bacterioplankton assemblage composition. Alteromonas-like organisms were never an important fraction of the assemblage, but Roseobacter dominated Alphaproteobacteria in Arenys Harbour. Furthermore, the bacterioplankton assemblages associated with Alexandrium spp. cultures were very different from the natural bacterial assemblages during blooms of the same species. We conclude that the presence of a given harmful algal bloom species during a bloom will not always necessarily be accompanied by the same bacterial assemblage structure, and studies done with dinoflagellate cultures may only reflect the bacteria capable of growing under laboratory conditions, with little resemblance to what occurs under natural conditions. © Inter-Research 2007.