Seasonal movements of 16 large (>400 mm total length) brown trout (Salmotrutta) were monitored for 216–347 d by means of radio telemetry techniques in Box CanyonReservoir, Washington. In 1992, radio-tagged fish moved out of the reservoir in late June whenwater temperatures reached 19–20°C and inhabited tributary streams containing water below16°C. However, similar movements into tributary streams were not observed in 1993 whenreservoir water temperatures were somewhat cooler. Diel tracking of fish in July–August 1992revealed that no fish returned to the reservoir during this period, but that half returned to thereservoir following their fall spawning season. The remaining fish returned to the reservoir duringhigh flows in March 1993. Summer residence of brown trout in Box Canyon Reservoir appearsclosely related to water temperatures. Brown trout survive in the reservoir by taking refuge incooler tributary streams when summer water temperatures are high.