Although numerous genetically isolated entities within Contracaecum rudolphii sensu lato are presently defined, information on the distribution and ecology of these groups is nonetheless in demand. In the present study, information based upon DNA sequence data (restriction analysis and sequencing of rDNA) on the distribution of the species C. rudolphii A and C. rudolphii B in the cormorant Phalacrocorax carbo sinensis from the Baltic region (Poland and Finland) is provided. These data corroborate previous work that identified C. rudolphii A in brackish water regions and C. rudolphii B in fresh water sites, although mixed infections also occur. The 2 species may inhabit the same host specimen. One rare heterozygote of the species was recorded.