Published data were used to compare the distance decay of similarity in parasite communities of three marine fish hosts: Atlantic cod Gadus morhua, the dab Limanda limanda and the flounder Platichthys flesus in two adjacent areas that differ with respect to the strength of a salinity gradient. In the Baltic Sea, which exhibits a strong salinity gradient from its connection with the North Sea in the west to its head in the north-east, parasite communities in all three fish hosts showed a significant decline of similarity with increasing distance. In contrast, among host populations in the North Sea, which is a fully marine environment, there was no such decline or only a weak relationship. The results suggest that environmental gradients like salinity can be strong driving forces behind patterns of distance decay in parasite communities of fishes. © 2010 The Authors. Journal compilation © 2010 The Fisheries Society of the British Isles.