The life cycle of Phflonema oncorhynchi was studied in sockeye salmon, Oncorhynchus nerka, from Cultus Lake, British Columbia. Gravid female worms from the coelom of sockeye spawners burst in lake water releasing living first-stage larvae. These were ingested by Cyclops bicuspidatus and developed to the infective third stage in the haemocoele. Development required 17 days at 12 C or 70 days at 8 C. Each of six hatchery-reared sockeye fingerlings were fed 14-70 copepods infected with third-stage larvae. Fourth-stage larvae were recovered from the peritoneal tissues of four fingerlings when examined four to ten days after infection. The later stages of development were studied by maintaining naturally infected sockeye salmon for two years in freshwater. These had early fourth-stage larvae in the parietal peritoneum and tunica adventitia of the swim bladder when captured as downstream migrants at Cultus Lake. When the fish were 26 months old, late fourth-stage larvae were found in the peritoneal tissues. These moved into the coelom when the fish were 32 months old and moulted to the preadult stage. The comparative morphology of mature worms collected from B.C. salmonids was studied. The type species, Philonema oncorhynchi Kuitunen-Ekbaum, 1933 was obtained from the type host, Oncorhynchus nerka, in the type locality, Vancouver, B.C. Philonema were also obtained from salmonids with a freshwater life cycle in a landlocked area, Kootenay Lake, which was once contiguous with the type locality of Philonema agubernaculum Simon and Simon, 1936. The morphology was constant for worms found in different hosts and geographical areas. Size was an unreliable characteristic and appeared to be a host-dependent variation. The type specimens of Philonema agubernaculum Simon and Simon, 1936 were examined and no differences in morphology found.