Adult nematodes of the family Cucullanidae, parasitic in the abdominal cavity of the brook lamprey, Lampetra planeri (Bloch) from the River Stensån, south Sweden are described and illustrated. According to the morphometry, these nematodes belong to the species Cucullanus truttae (Fabricius, 1794)--a common intestinal parasite of salmonid fishes. The name Dacnitis stelmioides Vessichelli, 1910 is considered to be a synonym of the latter. The role of lampreys in the life-cycle of C. truttae is discussed: although lampreys are either intermediate hosts or serve only as transport hosts for larvae of this parasite, the nematode larvae can attain maturity in this host.