To better understand the dynamics of diarrhetic shellfish poisoning (DSP) contamination a field study was carried out on the feeding behavior of Mytilus galloprovincialis (Lmk) during an important DSP outbreak. The study was focused on the relationships between phytoplankton in seawater and algal cells, or their remnants, in mussel stomachs. During the period studied, M. galloprovincialis seemed to feed selectively on dinoflagellates rather than diatoms. Further selection was observed among different dinoflagellate genera, a preference for the genus Dinophysis being particularly evident. In addition, mussels seemed to open the thecae of Dinophysis cells and digest them more easily than other dinoflagellates. Due to the high variability of the results of phytoplankton analysis in the mussel stomachs, no correlation was found between the abundance of Dinophysis species in the mussels' stomachs and the content of okadaic acid plus dinophysistoxin-1 in their digestive glands, evaluated with an ELISA assay. Conversely, the presence of Dinophysis fortii (the main DSP-causative agent in the area studied) in integrated net samples of the whole water column and the toxin content of the digestive glands presented similar temporal trends.