Background. A number of fish species in the Baltic Sea are known as definitive hosts for Pomphorhynchus laevis but it is unclear which of the Gammarus species is the intermediate host of this parasite. The aim of the present paper was to identify this host in brackish waters of the Baltic Sea. Materials and methods. A total of 531 scuds (G. salinus, G. zaddachi, and G. duebeni) were collected from the Gulf of Gdańsk and the Pomeranian Bay, respectively to determine the infection rate of those amphipods in the natural environment. Under experimental condition the scuds were exposed to infection with P. laevis in two different arrangements. In treatment one, the amphipods were kept, from May to July, in the same tank with infected flounder. In treatment two, 197 scuds were exposed to eggs of P. laevis, taken from dissected female acanthocephalans. Results. Scuds sampled from two areas of the Baltic Sea were not infected with P. laevis. Out of three Gammarus species cohabiting with infected flounders only G. zaddachi became infected. None of the scuds exposed directly to the eggs of the parasites became intermediate host of the acanthocephalan studied. Conclusion. Gammarus zaddachi is probably an intermediate host for Pomphorhynchus laevis in the Baltic Sea.