The incidence of Proteocephalus filicollis in Pungitius pungitius, Gasterosteus aculeatus f. hemigymnus and Gasterosteus aculeatus f. trachurus has been studied. It is very low in both forms of Gasterosteus, but 16% of Pungitius was infected. The monthly examination of minimally 27 and maximally 223 Pungitius pungitius indicates that an annual cycle exists neither in the incidence nor in genital development. It seems probable that Proteocephalus filicollis in Pungitius pungitius is less affected by low temperatures than worms of the same species in Gasterosteus aculeatus. Reference is made to intercurrent helminth infections.