Seabirds are prominent components of the North Atlantic marine environment, and their parasites offer an insight into seabird ecologic interactions. Parasites also provide vital information on historic biogeography of host associations and thus may reveal broad changes in the marine ecosystem. Helminths of Common Murres (Uria aalge) and Thick-billed Murres (Uria lomvia) in the northwest Atlantic marine environment were assessed to determine parasite community composition and changes in their parasite fauna since the 1960s. In total, 623 helminths, representing Digenea, Eucestoda, Nematoda, and Acanthocephala, were recorded from 100 Common and Thick-billed Murres collected from breeding colonies along the coasts of Labrador, Newfoundland, and Greenland. Parasite communities differed from those reported from the 1960s, and over 85% of the specimens were tapeworms (mostly in the genus Alcataenia). The high prevalence (26%) and mean intensity (14.6) of A. longicervica, a Pacific species recorded recently from Newfoundland, indicates that this tapeworm was established in the Atlantic by 2006. Significantly higher A. longicervica prevalence (>53%) and mean intensity (27.3) in the murres from Greenland and in wintering murres compared to murres from breeding colonies in Labrador and Newfoundland suggest a mechanism for the introduction of this species to the Atlantic. Periodic mixing of populations of Thysanoessa species, the euphausiid intermediate host of Alcataenia, occurs along the seas adjacent to the North Pacific and those along the Siberian Arctic. The mixing of infected Thysanoessa likely exposed North Atlantic and Arctic murres, which are geographically isolated from Pacific murres, to this tapeworm. The greater prevalence of A. longicervica in Thick-billed Murres was consistent with diet analyses, which revealed a greater proportion of euphausiids.