Since several years an alarming worldwide expansion of Harmful Algae Bloom (HAB) was observed in coastal and / or confined waters. Alexandrium catenella, a paralytic shellfish poisoning (PSP) producer, is now regularly found in Thau lagoon (south of France) at concentrations up to 15x106 cellules. l-1. In the same periods, events of oyster’s mortality are also observed causing huge economic losses. The purpose of this study is to investigate the relationship between these toxic events and mortality phenomenon. For that, we conducted a field work to evaluate the presence of the toxic A. catenella in Thau lagoon during the years 2013 and 2014 in which high mortality of oysters spats were observed in spring. At the same time, we experimentally infected spats with the pathogenic bacterium V. tasmaniensis LGP32 and kept them unfed or previously fed either with the toxic alga A. catenella or with nontoxic algae, Alexandrium tamarense or Tisochrysis lutea. Results showed that the toxic alga was present in the lagoon when spats' mortalities occurred suggesting A. catenella could be involved in these events. Moreover, our laboratory experiment showed that the exposure to A. catenella increases the susceptibility of spats to one of its pathogens, V. tasmaniensis LGP32. Those results both together suggest for the first time that, in the environment, toxic algae could be implicated in oysters’ mortality. Further studies should be conducted to determine the main factors of this alga implicated in weakening oysters and their immune system against their pathogen which lead to these large scale mortalities