Plasma concentrations of throxine (T4) in chinook salmon,Oncorhynchus tshawytscha, rearing at a New Zealand salmon hatchery, were measured by radioimmunoassay. Blood samples for assay were collected at approximately weekly intervals during the period February‐August (southern late summer‐winter). Plasma T4levels varied in a cyclical fashion, with maximum concentrations occurring near the time of each new moon. Within the lunar month, the mean date of the recorded T4peak was at 6.4 + 3.9 days before new moon (95% confidence level). Since thyroid hormones have been implicated in the process of salmonid smoltification, we suggest that the planning of hatchery release times for chinook salmon should include a component which takes account of the apparent link between lunar phase and circulating T4levels in these fish.