This study examined how social stress affects the disease resistance of chickens from various genetic lines. Aggression amongst the roosters was considered the cause of the social stress. The roosters were housed in individual cages. At 15 weeks of age, the aggression levels of pairs of roosters, from five genetic lines, were compared in an arena test. Based upon the results of aggressive encounters, the roosters were assigned to one of two categories, those being winners or losers. At 16 and 17 weeks of age, using the similar arena test, the aggression amongst pairs of roosters classified as winners was examined, similarly for roosters classified as losers. The duration of the aggression tests was 15 minutes and during said time the behaviour of the roosters was videotaped. The aggression amongst the roosters varied based upon their genetic line. Roosters that were selected for disease resistance showed fewer aggressive behaviours (pecks and fights) than the two lines that were bred to be susceptible to disease. The two recombinant lines (genes that have been altered at one region of the chromosome), exhibited similar aggression levels as the disease resistant roosters. Therefore, there may be a common gene or genes carried by the three genetic lines that result in the roosters expressing lower levels of aggression when they are provoked.