The lernaeopodid copepod Salmincola edwardsii is recorded for the first time in the British Isles as a parasite of the salmonid fish Salvelinus alpinus. Material from Ennerdale Water in the English Lake District is described and illustrated and evidence of the occurrence of the same species in four Scottish lochs is presented. An interesting biological feature of the Ennerdale population is that egg size varies almost three-fold in terms of volume in different individuals. Ennerdale's fauna is unique among those of British lakes in that it includes two crustaceans, Mysis relicta (known also from Ireland) and Limnocalanus macrurus (known nowhere else in the British Isles), both of which have been claimed as “ice-age” or “marine-glacial relicts”. No satisfactory explanation of how Ennerdale could have acquired these animals has hitherto been put forward. A hypothesis which attempts to do so and which also accounts for the presence of Salmincola on the char of Ennerdale and of certain Scottish lochs, but not on populations of this fish in other Lake District lakes, is put forward. This utilizes knowledge of the biology of the animals in question, such information as is available concerning the disposition of the ice in the area at the time of its retreat, and the record of the post-glacial history of the Cumbrian lakes as preserved in their sediments.