1. For two flocks of 48 and 40 hens, consisting of the F1 of Rhode Island Red males and White Leghorn females, peck-order, aggressiveness and egg production were determined. Flock I was built up step by step by continually adding a group of 8 hens to the hens already present in the testing pen. Flock II was brought into the testing pen as a whole at once. 2. The peek-order of the hens of flock I was clearly correlated with the date they had been put into the testing pen. The earlier the hens came into the testing pen, the higher their ranks. When the flock was rebuilt so that the groups were brought into the pen in reversed order, this correlation between peck-order and date of introduction into the pen was found again. 3. In flocks I and II a clear correlation between peck-order and aggressiveness existed, higher rank being correlated with stronger aggressiveness. 4. The peck-order, settled in flock I must have been determined for an important part by environmental factors, which can be summed up as “familiarity with the environment”. The peck-order of flock II must on the contrary have been mainly dependent on the physical and behavioural characteristics of the individual hens, because the environmental factors were equal here for all hens from the beginning. The hens were all brought into the testing pen together. 5. In flock I a clear correlation existed between peck-order and egg production. Hens with a higher rank laid more eggs and produced a higher total egg weight. The same correlation exists between aggressiveness and egg production. A stronger aggression is correlated with a higher egg production. 6. The correlation between peck-order and aggression on the one hand and the egg production on the other hand runs through the feeding possibilities. The less aggressive hens get less food, because they are regularly chased from the food trough by the more aggressive hens. This shortage of food leads to a decreased egg production. 7. For flock II no clear correlation could be shown between peck-order and aggressiveness on the one hand and the egg production on the other hand. It is probable that the egg production of this flock started during the experiment and, therefore, was not yet properly stabilized.