I. 240 Male mice from the aggressive C57BL/I05 strain and non-aggressive A/J strain were subjected to temperatures of I8.3°C, 2I.I°C, 26.7°C, 32.2°C or 35°C for 24 hours. 2. The mice were then crowded in groups of 8 into enclosures with floor areas of 0.09, 0.2I, or 0.38 m2. 3. The frequency of aggression acts (biting frequency) was recorded for each animal 30, 60, and 90 min. after being put in the enclosures. 4. Aggression was lowest at I8.3°C and 35°C and tended to increase as the temperature increased from I8.3°C to 32.2°C. 5. There appeared to be more aggression in the more crowded conditions than in the less crowded conditions. 6. The results were attributed to metabolic changes induced by increasing temperature and to perceptual and proximity effects resulting from increasing population density.