Specimens of flounder Platichthys flesus were randomly sampled from three regions of the southern Baltic Sea: the Gulf of Gdańsk, the Pomeranian Bay, the open sea off Łeba and from the northern Øresund, Kattegat. The collected parasites were morphologically identified and their genotypes at seven loci coding for enzymes were checked. Wide ranges in both the numbers of rows of rostellar hooks and the numbers of rostellar hooks per row were detected, indicating the presence of two species in the material: Pomphorhynchus kostylewi and Pomphorhynchus laevis. However, electrophoretic data does not support occurrence of two independent gene pools.