Innate species-specific temperature preferences of fish are subjected to fluctuations under a variety of environmental, physiological, and developmental conditions. The temperature preference patterns of two ecologically distinct races of chinook salmon (Oncorhynchus tshawytscha) were investigated as laboratory held animals underwent smoltification. Smoltification is a distinct developmental stage in the life history of anadromous salmonids when juvenile fish undergo profound behavioral, morphological, and physiological differentiation prepatory to seawater entry. A group of spring and fall chinook salmon were held under identical conditions of increasing water temperature over the course of smoltification. Another group of spring and fall chinook salmon were held at a constant water temperature of 8° C over the same period of time. Changes in the preferred temperature of juvenile chinook salmon were associated more closely with the size (fork length) of fish than with time (days). Both spring and fall chinook salmon held at 8° C showed an increase in thermal preference of about 1° C as fork length increased in these respective groups. This increase in thermal preference is thought to be thermoregulatory, accelerating smolt development in fish held in inhibitory low water temperatures. Spring chinook salmon held at increasing water temperatures showed no change in thermal preference associated with smoltification. Fall chinook salmon held at increasing water temperatures displayed a large drop in thermal preference towards the end of smolt development. Differences in the thermal preference patterns of spring and fall chinook salmon during smoltification may result from local habitat adaptations, as well as seasonal differences in smolt migration.