Larvae of a species of Proteocephalus were observed in the intestines of larvae of Sialis lutaria (Insecta: Megaloptera) in a single locality at prevalence levels of up to 40% and over a period of three years. The cestodes were plerocerciform in character and up to 3 mm in length, having undergone some growth and development in the insect. Comparison of scolex morphology with that of adult Proteocephalus spp. obtained from fish in the same river suggests the most likely identity of the parasite is P. filicollis. Support for this suggestion came from the recovery of two parasites from experimentally infected sticklebacks, Gasterosteus aculeatus. This is the first record of a proteocephalid larvae from an insect, and so also from S. lutaria, and of a proteocephalid using a second, additional invertebrate host. The use of S. lutaria is so far known only from this one locality and may reflect the peculiar, but continuing, ecological conditions there, but it is suggested that the ability to use S. lutaria as an additional host may be associated particularly, though not exclusively, with parasites of sticklebacks.