During the last two decades the potentially toxic dinoflagellate Prorocentrum minimum (PAVILLARD) SCHILLER has successfully established itself in the Baltic Sea. It is now a dominant summer species in the southern Baltic proper, and reaches as far into the low salinity of the northern Baltic as the central Gulf of Finland. In the 1990s, it developed several coastal blooms in the eastern and northern Baltic proper, but occurred irregularly between years. Field data show that P. minimum can grow at salinities below 5 PSU, confirming its potential to penetrate farther into the low saline part of the Baltic Sea. Biometric data show that P. minimum cells differ significantly in size between areas in the Baltic Sea.