Animals are marked for identification in a range of experiments from behavior and wildlife management, to reproduction and pharmacological studies. However, little is known of the impact marks have on animals. The objective of our experiment was to determine the impact of marking on aggression and stress in the domestic fowl. Broilers, in groups of 10 and 50 were housed with 20%, 50% or 100% of the birds marked. Aggressive interactions, given and received, were observed from 3 to 10 wks. Our results revealed that marked birds received significantly more aggression than unmarked birds, and subsequently deliver fewer aggressions to pen mates. Marked birds appear more stressed than their unmarked pen mates, especially in 20% pens. Marked birds in 20% pens also had a lower epinephrine response to manual restraint. Our findings show that marks can impact both the aggressive behavior and stress of the birds bearing them.