Lampetra macrostoma n.sp., a freshwater parasitic lamprey, is distinguished from related species L. tridentata, L. lethophaga, L. folletti, L. minima, and L. similis by its parasitic habit and very large disc. Other characters distinguishing the species from L. tridentata are its longer prebranchial length, large eye, weakly pigmented velar tentacles, and its ability to remain in freshwater. The recently metamorphosed form readily survives in freshwater and probably is non-anadromous even though it can survive in salt water. The new species has been discovered in two lakes on Vancouver Island, British Columbia, Canada, where it attacks large numbers of resident salmonids. Because of its ability to survive and feed in freshwater, it poses a definite threat to freshwater fishes.Key words: lamprey, new species, non-anadromous lamprey, salmonid parasitism
