During toxic spring and fall blooms produced by the dinoflagellate Alexandrium in the Thau lagoon (Mediterranean Sea), we monitored the presence of Amoebophryidae (Syndiniales), a group of parasites virulent toward a wide range of dinoflagellate hosts. A PCR-biased approach unveiled the presence of at least 10 different parasitic groups during Alexandrium proliferation. However, fluorescent in situ hybridization failed to reveal parasitic infection inside Alexandrium cells in field populations. In contrast, several co-occurring, less abundant thecate dinofiagellate species were infected by Amoebophryidae, showing up to 10% of infected cells. We concluded that Alexandrium populations were not infected by these local parasites, at least during our survey. In order to check this resistance capacity on a more global scale, we cross-infected several Alexandrium strains isolated from the Thau lagoon with one strain of the parasite Amoebophrya sp. originating from Salt Pond, MA, USA. All of these hosts were strongly infected by the North American parasite, leading to the conclusion that blooming Alexandrium in the Thau lagoon were not particularly resistant to this kind of parasite. These results provide additional evidence that dinofiagellates may become invasive when they successfully escaped their natural enemies in time and/or space (the ``enemy release'' hypothesis). (C) 2011 Institut Pasteur. Published by Elsevier Masson SAS. All rights reserved.