The purpose of this study was to describe the composition and occurrence of species of thecate dinoflagellates potentially toxic and harmful in cultured shellfish, located at Easel Itapocorói (AI) and Praia Alegre (PA) in the municipality of Penha, SC. Water samples and environmental parameters (salinity, water temperature and measured Secchi disk) were collected weekly during the months of April 2007 and April 2008 at two sites sampled by monitoring maintained by the Laboratory for the Study of Harmful Algae, CTTMar, Univali. The identification of the bodies was through an Olympus microscope, model BX41 with up to 1000x. During the sample period, we identified 21 species of thecate dinoflagellates potentially toxic and harmful in the genus Alexandrium, Ceratium, Dinophysis and Prorocentrum. The species P. obtusum (86%), D. acuminata (78.5%) and P. micans (72%) were very frequent throughout the period in the two sample points. The point with highest species richness was the AI, with 14 species, and the PA had the lowest point in two species richness. Most of the samples was considered rare relative abundance of thecate dinoflagellates compared to the presence of harmful micro algae, but during the month of September, were considered abundant, during which there was a flowering of D. acuminata. The similarity between the two points throughout the sample period had a high coefficient (86%), and between the months examined, the month of June 2007 showed the lowest similarity (10%) and the month of July 2007 the greatest similarity recorded (91%). The composition and distribution of thecate dinoflagellates during the study showed that the existence of a dynamic community structure could be used as a tool to aid the monitoring of harmful algae mariculture activity in this region.