morphological studies, experimental cross-infections and host specificity data suggest that the species of swimbladder nematode which matures in coregonus clupeaformis, from inland lakes is not distinct from cystidicola farionis which matures in coregonus artedii and other salmonids from lakes superior and nipigon. adult worms in coregonus clupeaformis are morphologically indistinguishable from those in other hosts. however, eggs of worms in coregonus clupeaformis have predominantly lateral filaments, while those in other hosts have predominantly polar filaments. larvae which accumulate but do not mature in the swimbladder of coregonus clupeaformis from lake nipigon, grew and developed toward maturity in hatchery-reared coregonus clupeaformis of lake simcoe origin. one, mature, male worm was found 77 days after experimental infection. only female worms containing unshelled eggs were found after 120 days. this suggests that the development of c. farionis may depend on host strain. uncertainty remains as to whether variations in egg morphology are host-induced or due to genetic differences of the worms involved. filaments on the eggs of c. farionis readily adhere to the setose areas on the appendages of amphipod intermediate hosts. presumably, this increases the probability of the intermediate host ingesting eggs. eggs stored in water or saline for periods of two weeks or more do not hatch in the gut of amphipods. only eggs recently released from female worms hatch and develop. mature swimbladder nematodes were found in coregonus clupeaformis from all of seven inland lakes examined in northwestern ontario. coregonus artedii was present in three of these lakes but was not infected, perhaps due to the lack of amphipod intermediate hosts in its diet. cystidicola farionis matures in salvelinus namaycush from pettit lake, ontario and has eggs resembling those from worms in coregonus clupeaformis from the same lake. prosopium coulteri from lake superior are infected with c. farionis (new host record) but are not considered to be a suitable host for the worm as no mature worms were found. infections in coregonus clupeaformis from pettit lake and in salmo gairdneri from lake superior exhibited similar seasonal periodicity with largest worm numbers occurring in the spring. the cephalic structures of c. farionis and its close relative, c. stigmatura, were studied using a scanning electron microscope and are compared with those of other cystidicolids.