Specimens of £ounder Platichthys £esus were randomly sampled from three regions of the southern Baltic Sea: the Gulf of Gdan¤ sk, the Pomeranian Bay, the open sea o¡ L Àeba and from the northern Òresund, Kattegat. The collected parasites were morphologically identi¢ed 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.