The present paper deals with investigations on the occurrence in Netherlands' fresh water fishes and host specificity of tapeworms belonging to the genus Proteocephalus. The species involved are:

P. tetrastomus (Rudoiphi, 1810) in smelt, Osmerus eperlanus.

P. longicollis (Zeder, 1800) in smelt, Osmerus eperlanus.

P. cernuae (Gmelin, 1790) in pope, Acerina cernua.

P. ocellatus (Rudolphi, 1802) in perch, Perca fluviatilis.

P. macrocephalus (Creplin, 1825) in eel, Anguilla anguilla.

Seasonal cycles were found in P. telrastomus and P. ocellalus; in the latter species this was certainly induced by low winter temperatures.

Experimental infections showed that a high degree of host specificity exists in all species considered. Infections of an alien host were never successful. Two kinds of infections were carried out: (a) feeding plerocercoids (in the intermediate host), coming from a known source, to a number of fishes, belonging to several species including the original host species, and (b) transplanting worms, collected from the intestine of a fish, to the intestines of a number of other fishes, belonging to several species including the original host species.

Notwithstanding the high degree of host specificity, worms may survive for one or two days, but usually in a bad condition, in the intestine of an alien host. Occasionally they survived up to 14 days: e.g. P. tetratomus in Anguilla anguilla. Consequently sometimes dissections of predator fishes or carrion eating fishes may produce dubious host records because actually Proteocephalus from their prey are recorded. We are convinced that P. tetrastomus found in Salmo trutta, Platichihys flesus and Perca fluviatilis originate from Osmerus eperlanus having been eaten by these species.

Additionally cannibalism may be responsible for the fact that large specimens of some fish species, e.g. smelt, are heavily infected with Proteocephalus while they hardly eat any Cyclops, the intermediate host. This is clearly demonstrated by the transplantation experiments, transplanting worms to the proper host.

From the data recorded it is apparent that representatives of the genus Proteocephalus are not harmful to commercially important fishes in the Netherlands. In these fishes natural infection with the proper species of Proteocephalus is low and successful infection with other species, showing a high incidence in less valuable fishes, is prevented by the high degree of host specificity. Additionally worms of the species studied did not injure the tissues of the intestinal wall by the action of their suckers and piercing of the intestinal wall never occurred. Even smelts infected with large numbers of worms-more than 50 in a single 20 cm. specimen-did not show any abnormalities in genital development or in the amount of fat in the body cavity.