This paper is a descriptive synthesis of approximately 17 months of phytoplankton sampling in the shellfish culture area of Enseada de Armação do Itapocoroy (Penha - Santa Catarina - Brazil). The goal of this program was to know the local phytoplankton composition, specially the toxic and potentially toxic ones; to verify the characteristics of water masses associated to the different phytoplankton assemblies and to monitor the occurrence of blooms of toxic or potentially toxic species, in order to prevent human intoxication by ingestion of contaminated shellfish. The results have indicated the occurrence of seasonal patterns in species succession. Potential producers of diarrhetic toxins tend to occur in winter and spring. From late spring to late summer the occurrence of Trichodesmium spp. indicate the possible presence of cyanobacteria toxins. Alexandrium fraterculus - a potential producer of paralytic toxin - has ocurred only in low cell densities and mainly in winter.