In a transplant experiment, two geographically distinct populations of Mytilus edulis with different histories of contamination by Paralytic Shellfish Poisoning (PSP) toxins were exposed to natural blooms of the toxic dinoflagellate Alexandrium excavatum.  Transplanted mussels were suspended in cages from the quai at the site of the experiment. Mussel and Alexandrium cells samples were collected periodically and their PSP toxin concentration and composition analysed by High Performance Liquid Chromatography (HPLC). The mussels encountered two successive blooms of A. excavatum differing in duration, maximum cell concentration and toxicity per cell. The shorter first bloom displayed cell concentrations an order of magnitude greater than the second bloom, but the toxicity of the cells increased by a factor of four during the latter. The two populations displayed different PSP toxin accumulation and elimination patterns during and after the first but not the subsequent bloom. During the first bloom, the maximum toxin concentration of chronically exposed mussels was twice that of the pristine mussels, but in terms of toxicity, the difference was less pronounced.  Putative toxin transformation was examined in the two populations, based on comparisons of toxin profiles in Alexandrium cells and in mussel digestive glands and by comparison of temporal changes in toxin epimeric ratios in the two mussel populations. Prior exposure to toxic Alexandrium blooms appeared to have an effect on transformation of PSP toxins. During the first bloom, the toxin patterns of pristine mussels resembled those of the Alexandrium cells, but following a first exposure, the two mussel groups had comparable toxin patterns.  Mussels which had not been pre-exposed to PSP toxins, seemed to accumulate less toxin on a total molar basis in the presence of high Alexandrium cell concentrations, but they contained a higher proportion of highly toxic derivatives, and detoxified more rapidly than previously exposed mussels