the development of cystidicola cristivomeri in lake trout, salvelinus namaycush, and c, farionis in rainbow trout, salmo gairdneri, is described. the population biology of c. cristivomeri was investigated in lake trout in three lakes in northwestern ontario and in arctic char, s. alpinus, gaviafaeces lake, northwest territories. young lake trout fed selectively on large mysis relicta which were more frequently infected with c. cristivomeri (up to 5.1%) than small mysids. pontoiporeia affinis was not a suitable intermediate host in nature and there was no evidence that fish paratenic hosts were important in transmitting this nematode to lake trout. third-stage larvae given to lake trout migrated directly via the pneumatic duct to the swimbladder. in experimentally infected fishes, at 4-10°c, c. cristivomeri were mature after 67 (males) and 210 days (females); c. farionis were mature after 112 (males) and 235 days (females). there was no measurable mortality of c. cristivomeri after 600 days in experimentally infected lake trout. field studies indicated that most c, cristivomeri live at least 10 years and some probably live longer. the development of female worms to sexual maturity was retarded and they grew more slowly when large numbers of c, cristivomeri were present in the swimbladder. short female worms produced eggs at a slower rate than longer females. this density dependent regulation of c, cristivomeri at the infrapopulation level may result in long-term stability of the nematode suprapopulation in a lake. the number of ulcerative lesions on the inner surface of the swimbladder of lake trout was directly dependent upon the number of mature c. cristivomeri present in a fish.