Mytilopsis leucophaeata (Conrad, 1831) is native to the western Atlantic coast of North America. It has been reported in many regions outside its home range, including the Gulf of Gdańsk (the southern Baltic Sea). Although the conditions in the area seem to be favorable for the species, there have been no reports about its presence elsewhere in the gulf. However, the data probably did not reflect its actual distribution in the region, as the species can be easily misidentified for other bivalves living in the area, and it prefers hard substrates, which have not been studied as extensively as soft substrates. Our study was aimed at determining the actual distribution of M. leucophaeata in the Gulf of Gdańsk, the Vistula Delta and the Vistula Lagoon. The results showed that the species was not widely distributed in the Gulf of Gdańsk and was absent in the Vistula Lagoon. The only new records come from the Vistula Delta where the abundance of M. leucophaeata was up to 69 ind. m-2, i.e. significantly higher than in the Gulf of Gdańsk. Individuals found in the Vistula Delta were also significantly larger, with the length of shells reaching 22-23 mm, which is considered to be the maximum for the species.
