Brown trout, Salmo trutta, were allowed to thermoregulate individually in an electronic shuttlebox. Pooled data for 6 fish showed a diel pattern of preferred temperature, with a diurnal minimum of 10 .3°C, an early nocturnal maximum of 13 .7°C, a less pronounced mid-scotophase minimum of 11 .7°C, and a secondary dawn maximum of iz .8°C, in a somewhat crepuscular pattern . The 24-hour mean preferendum was 12 .2°C .