Young bulls were finished over a period of 8 months. They were either mixed or unmixed and were grouped with similar or non-similar BW at the beginning of the finishing period. Social behavior following mixing and throughout the finishing period, stress measurements (cortisol concentration, adrenal weight, catecholamines-synthesizing enzyme activity, and pain sensitivity), and production traits such as growth rate, homogeneity, feed efficiency, and carcass measurements were analyzed. Frequency of social interactions among the bulls increased after mixing. These included agonistic and nonagonistic interactions as well as sexual interactions. Agonistic and sexual interactions were more frequent among bulls in the similar BW groups. During the finishing period, synchronization of activity tended to be lower among mixed bulls. At slaughter, the activity of the catecholamines-synthesizing enzyme, tyrosine hydroxylase, was increased in mixed bulls, possibly indicating chronic stress. There were no differences noted in ADG or carcass traits among treatments. Body weight variability within groups increased in groups that initially had similar BW, but decreased in groups that initially had nonsimilar BW. Conclusions stated that mixing young beef bulls at the beginning of the finishing period did not have detrimental effects on health and production but mixing did increase stress levels, possibly due to decreased cohesion within the groups. The authors did not confirm that there was any benefit to mixing bulls at the onset of finishing in order to create groups of similar BW. This was because BW similarities within groups were not maintained throughout the finishing period.