In the context of the French phytoplankton and phycotoxins monitoring network (REPHY), shellfish scallops were harvested systematically during the authorised fishing season for the purpose of investigating paralytic toxins (PSP-mouse test), amnesic toxins (ASPchemical analysis), and lipophilic toxins (DSP-mouse test). For all shellfish samples that tested positive in the DSP mouse bioassay used for lipophilic toxins detection, liquid chromatography coupled with mass spectrometry (LC-MS/MS) was used to search for the following lipophilic toxins: okadaic acid, dinophysistoxins, pectenotoxins, azaspiracids, yessotoxins, spirolides, and gymnodimine. In order to investigate the presence of okadaic acid esters (DTX3), alkaline hydrolysis was performed on all samples, with LC-MS/MS analyses being applied to the samples before and after hydrolysis. During 2004–05, the results revealed two consecutive contamination periods of French scallops (the Bay of Seine, Normandy): firstly by domoic acid for the duration of the 8 months from November 2004 until to June 2005, and secondly by okadaic acid and DTX3 associated with Dinophysis up until the month of December 2005. This paper reports on the first occurrence of domoic acid, okadaic acid, and DTX3 in French scallops. Paralytic toxins, however, were not detected.