We evaluated a stock for restoring runs of fall Chinook salmon Oncorhynchus tshawytscha in the Upper Klamath River basin by monitoring its development in Iron Gate Hatchery and in net-pens in the Williamson River and Upper Klamath Lake in Oregon. We transferred age-1 hatchery fall Chinook salmon to net-pens in October 2005 and age-0 fall Chinook salmon in May 2006. Indices of smolt development were assessed in the hatchery and after 3 and 14 d in net-pens. Based on gill Na⁺, K⁺-ATPase activity and plasma thyroxine (T4) concentration, age-1 Chinook salmon were not developing smolt characteristics in the hatchery during October. Fish transferred to the river or lake had increased plasma cortisol in response to stress and increased T4 accompanying the change in water, but they did not have altered development. Variables in the age-0 Chinook salmon indicated that the fish in the hatchery were smolting. The fish in the river net-pens lost mass and had gill ATPase activity similar to that of the fish in the hatchery, whereas the fish transferred to the lake gained mass and length, had reduced condition factor, and had higher gill ATPase than the fish in the river. These results, along with environmental variables, suggest that the conditions in the lake were more conducive to smoltification than those in the river and thus accelerated the development of Chinook salmon. No Chinook salmon in the hatchery or either net-pen became infected with the myxosporean parasite Ceratomyxa shasta (the presence of which in the river and lake was confirmed) during either trial or when held for 90 d after a 10-d exposure in net-pens (2006 group). We concluded that that there is little evidence of physiological impairment or significant upriver vulnerability to C. shasta among this stock of fall Chinook salmon that would preclude them from being reintroduced into the Upper Klamath River basin.