The study aims were to quantify the survivorship of Anguillicola crassus in eels in seawater and investigate transmission in estuarine (50% seawater) and marine (100% seawater) simulated conditions. Most A. crassus were able to survive and reproduce in 50% and 100% seawater eels for up to 6 months and therefore could survive during the spawning migration of eels to the Sargasso Sea. Up to 10% of the parasite metapopulation were damaged after long-term exposure to 50% or 100% seawater. Transmission of A. crassus was completed in 50% and 100% seawater in the laboratory, although infection levels in 100% seawater hosts were always lower. Transmission in estuaries is ecologically possible since the copepod Eurytemora affinis is susceptible to infection and is the dominant autochthonous species in most tidal estuaries in the Northern hemisphere. Transmission at sea is unlikely due to lack of suitable intermediate hosts. The data support the suggestion that dissemination of A. crassus has been facilitated by natural movement of eels in coastal waters. It is possible that A. crassus could impair the success of the eel spawning migration to the Sargasso Sea, although there is no direct evidence to support this.