The factors contributing to the similarity between digenean parasite assemblages parasitizing the black-headed gull were investigated. Thirteen different digenean species from the digestive tract and the bursa of Fabricus were found. We predicted that the structure of parasite communities is not random at either component community or infracommunity levels. We searched for nested patterns in the digenean community, and explored the possible factors contributing to nestedness. We found that digenean species which occupy a narrow range of intermediate hosts are placed out of order in the nested matrix. The influence of several variables related to the water reservoir, geographical distances, and the abundance of intermediate hosts on the species diversity and similarity of parasite communities were tested as well. Because of the complexity in bird digenean life-cycles we supposed that intermediate hosts and ecological characters of their environment could play a major role. We showed that the presence of intermediate hosts is the factor limiting the qualitative and quantitative similarity of parasite communities among different host populations as well as influencing the digenean species diversity in the definitive host. The similarity in abundance of intermediate hosts between different localities was facilitated by the presence of vegetation in water reservoir and more diversified type of water bottom. Digenean species diversity was higher when the water reservoir was exposed to temporal drying. We hypothesized that this factor could facilitate the inclusion of molluscs infected by digeneans in the diet of birds. Both species diversity and dominance were influenced by the bottom type of water reservoir and temporal drying up. Geographical distances between localities influenced the quantitative similarity of digenean communities as well as species richness.