In total, we analyzed over 4000 individuals of 12 gammarid species from the deltaic system of the Vistula River, Vistula Lagoon, littoral of the Baltic Sea near Dqbki, as well as from small rivers draining directly to the sea in this area. Gregarines (Apicomplexa, Gregarinidae) were recorded in digestive tracks of invasive Pontogammarus robustoides (Uradiophora ramosa (Balcescu-Codreanu 1974) and Cephaloidophora mucronata (Codreanu-Balcescu 1995)) from the Vistula deltaic system, and in digestive system of native Gammarus pulex (Cephaloidophora gammari (Franzius 1848)) from Stradanka river, an affluent of the Vistula Lagoon. Microsporidia were found also only in the two above gammarid species: Pleistophora muelleri (Pfeiffer 1895) in G. pulex from Piainica river, and Nosemapontogammari (Ovcharenko and Kurandina 1987) from the Vistula deltaic system. All the above microparasites are new to Poland.