The role of temperature in territory selection and the modifying influence of population density and prior residence were studied with bluegills (3.3-5.3 cm total length) in 57-liter bichambered shuttle aquaria. One side of each aquarium was held at the preferred temperature of 31 C and the other side was lower. In 13 out of 14 cases, the dominant fish in groups of 4 fish set up a territory on the side with 31 C rather than 26 C. With different population densities territorial behavior displaced subordinates from the side with 31 C at low densities (2 fish per aquarium), but results were variable at intermediate densities (4 or 8 fish per aquarium) and at high densities (16 or 32 fish per aquarium) almost all fish crowded into the side with 31 C. Larger differences in temperature produced more crowding on the side with 31 C. In populations of 8 fish, prior residence was more important in territory selection than a 5 C difference in temperature over a short time period. In conclusion, temperature can influence selection of territories and at low densities social behavior can exclude subordinates from