Long-term, high frequency studies of phytoplankton (0-20 m) in a coastal and open-sea area of the brackish northern Baltic Sea proper, were used to describe long-term and seasonal variability of Dinophysis acuminata and D. norvegica. In addition, we also studied their vertical distribution. The longest time series (1977-1998) are from two coastal stations, one exposed to the open Baltic Sea, the other from a large, less exposed bay (Himmerfjärden) enriched in nutrients. Since 1990 data are also available from an open sea station (The Landsort deep, 450 m ˆ the deepest part of the entire Baltic Sea).Our data show similar longˆterm behaviour of D. norvegica at the two coastal stations. This species was virtually absent in the seventies, increased during the eighties and was commonly present in high abundances during the early nineties. The open sea station behaved similar to the coastal station during the nineties. D.acuminata showed no clear long-term changes in abundances, but at the bay station, abundances were considerably higher compared to the other two stations. In contrast, abundances of D. norvegica decreased substantially from the open sea to Himmerfjärden Bay. We also found a clear seasonal as well as depth separation in the occurrence of D. norvegica and D.acuminata. Typically D. norvegica peaked later in summer than D.acuminata and mainly occurred close to the summer thermocline while D.acuminata occurred shallower, where light were more abundant.Despite large between year and station variability, these were not clearlyrelated to environmental conditions. However, the differences in distribution of D. norvegica and D. acuminata are likely explained by D. acuminata tolerating lower salinities.