the life cycle and larval development of swim bladder nematodes, cystidicola spp., have been elucidated. c, farionis developed in three species of amphipod. naturally infected amphipods have been found previously but one well known report was not valid. c. cristivomeri. developed in mysis relicta but not in amphipods. naturally infected mysids were found. the third-stage larva of cystidicola spp. and other cystidicolids undergoes considerable growth and development, but not the third moult, in the intermediate host. c, cristivomeri was found in lake trout, brook trout, a new host record, and lake whitefish, probably an abnormal host. the intensity of infection of c. cristivomeri in lake trout in northwestern ontario lakes was very high where the trout may be planktonivorous throughout their life. c, cristivomeri may have disappeared from the great lakes when lake trout populations declined severely. c. farionis was found in a variety of salmonids including introduced species in lakes huron and superior. eggs of c. farionis had mostly lateral filaments in lake whitefish and mostly polar filaments in other fish hosts. the number of filaments was variable. changes in the intensity of infection in lake whitefish with age likely relate to changes in amount and kind of food eaten and growth rate of the fish. the movements and feeding habits of the fish hosts determine which amphipods are important in natural transmission of the parasite.