Introduction Cloacotaenia megalops (Nitzsch in Creplin, 1892) is a tapeworm, distributed all over the world, from the family Hymenolepididae [1]. Its generic name is derived from the typical location in the cloaca of final hosts, birds associated with water and marsh environment, i.e. in Anseriformes and much less frequently in Galliformes and Gruiformes [1]. Over the past several dozen years, reports have used several names of this parasite species, with the best known: Hymenolepis megalops (Creplin, 1829), Orlovilepis megalops (Creplin, Nitzsch et al., 1829) and Taenia megalops (Creplin, Nitzsch et al., 1829). The natural intermediate host of this tapeworm is an ostracod Cypris pubera [2] although cysticercoids have been experimentally obtained from both C. pubera [3] and Heterocypris incongruens [4]. In Poland, the presence of C. megalops has been reported in 16 species of ducks from the Baltic coast, the Mazury Lake district, the Wielkopolska- Kujawy Lowland, Mazovia Lowland and Podlasie [5]. In Western Pomerania, quantitative analysis of cestodofauna has been carried out only in Anas platyrhynchos [6], therefore the aim of this study was a wider ecological analysis of C. megalops populations, involving 17 species of free-living Anatinae ducks in northwestern Poland. Original papers Cloacotaenia megalops (Nitzsch in Creplin, 1829) (Cestoda, Hymenolepididae) in wild ducks in Western Pomerania 1 Katarzyna Królaczyk 1 , Katarzyna M. Kavetska 1 , Elżbieta Kalisińska 2 , Małgorzata R. Nowak 1 1 Laboratory of Biology and Ecology of Parasites, 20 Judyma Street, West Pomeranian University of Technology, 71- 466 Szczecin, Poland 2 Department of Biology and Medical Parasitology, Pomeranian Medical University, 72 Powstańców Wielkopolskich Av., 70-111 Szczecin, Poland Corresponding author: Katarzyna Królaczyk; E-mail: katarzyna.krolaczyk@zut.edu.pl ABSTRACT. Cloacotaenia megalops (Nitzsch in Creplin, 1892) is a polyxenic and cosmopolitan tapeworm from the family Hymenolepididae. Its generic name derives from their typical location (cloaca), and the typical final hosts are birds typically associated with water and marsh environments: Anseriformes, Galliformes and Gruiformes. In Poland, the presence of C. megalops has been observed so far in 16 species of ducks from the Baltic coast, the Mazurian Lakeland, Wielkopolsko-Kujawska Lowland, Mazovian Lowland, and Podlasie Lowland. In Western Pomerania, quantitative structure analyses were only carried out on Anas platyrhynchos, and therefore the aim of this study was the detailed analysis of environmental populations of C. megalops in wild ducks. The examined tapeworms were isolated from the digestive tract of 1005 wild ducks representing 17 species belonging to three different eco-tribes: Anatini (n=225), Aythyini (n=413) and Mergini (n=367), from northwestern Poland. During the study, 187 tapeworms were found in 89 birds (8.8% of examined ducks) belonging to 7 species: Anas crecca (common teal), A. querquedula (garganey), A. platyrhynchos (mallard) (Anatini); Aythya ferina (pochard), A. fuligula (tufted duck), A. marila (greater scaup) (Aythyini) and Bucephala clangula (goldeneye) (Mergini). The results show the differences in the quantitative structure of C. megalops among the examined species of ducks. The highest prevalence was found in mallards (18.6%) and the lowest in greater scaup (3.2%). The highest mean intensity was observed in greater scaup (4.0), and the lowest in garganey and common teal (1.0). Relative density was at a similar level in the tested birds. Based on the ratio of dominance, it was found that C. megalops is a rare species in the cestodofauna in the examined birds. 1 This study was supported by Ministry of Science and Higher Education, grant no. Wiadomoœci Parazytologiczne 2011, 57(2), 125–128 Copyright© 2011 Polish Parasitological Society