The duration of infections with Myxidium salvelini, a freshwater myxosporean, in the kidneys of anadromous sockeye salmon (Oncorhynchus nerka) smolts from Cultus Lake, Fraser River system, British Columbia, was followed both in fresh water (FW) and in sea water (SW). The smolts were collected at the outlet of the lake as they migrated seaward, were subsequently held captive in M. salvelini-free FW or SW, and were sampled for presence of the parasite biweekly. Initial prevalence of infection was 80% or higher. In FW-held fish, spores were detectable over the 28-wk period of sampling, but the prevalence of infection with spores and prespore forms declined sharply after 22 wk. Nine weeks after the fish were transferred to SW, spores no longer were present but prespore forms continued to be present at high prevalences for up to 25 wk. SW-held fish were reacclimated to FW 4 wk after the last detection of spores. Spore production was resumed in this group of fish within 8 wk and continued for the next 5 wk, when the study was terminated. This indicates that M. salvelini persisted in an arrested prespore form during the SW holding period. This is the first report of arrested development in a prespore stage of a myxosporean. The physiological changes in the kidney that accompany migration of anadromous salmon from fresh to sea water likely inhibit spore production in M. salvelini. In FW salmonids, M. salvelini appears to have an annual life cycle.