The Comacchio lagoonal system has been affected for years by a persistent phytoplanktonic bloom which makes the waters greenish. Surveys carried out from April 1995 to February 1996 showed that this bloom was caused by a Synechococcus sp. and by an Eustigmatophycean belonging to the genus Nannochloropsis. However, while the Synechococcus sp. was mainly present in the summer, Nannochloropsis was always present with constant density values. Moreover, this species shows many ultrastructural features and a pigment composition similar to those of Nannochloropsis spp. and reproduces by autospores. The 18S rRNA gene sequence of this strain is identical to those of five strains of Nannochloropsis gaditana. In this paper, besides phytoplankton (including picophytoplankton), we also report the data of the main chemico-physical parameters. Among them, the salinity and nutrient values showed an anomalous seasonal trend.