Abstract This study tested the hypothesis that thyroid hormone levels in wild juvenile coho salmon Oncorhynchus kisutch are higher in fish beginning their seaward migration (smolts) than in lake-resident fish (parr). Average overall thyroxine (T4) levels were higher in wild smolts (3.5 ng/mL) than in wild parr (1.8 ng/mL); however, average overall triiodothyronine (T3) levels were similar in wild smolts (3.0 ng/mL) and parr (2.3 ng/mL). Whereas hormone levels in smolts varied between sampling dates (mean T4, 1.2–6.6 ng/mL; mean T3, 0.7–4.8 ng/mL; P 0.05). Partly because of these dissimilar patterns of variability, differences in thyroid hormone levels between smolts and parr tended to be subtle. Thus, wild coho smolts were more similar to wild parr than might have been predicted from studies of hatchery-reared fish. Furthermore, because hormone levels in migrants were sometimes similar to those in...