Commercial broiler breeder males are reported to display high levels of aggression, often injuring and sometimes killing females. Aggression toward females by mature male domestic fowl is rare and may be regarded as aberrant behaviour. The objective of this study was to determine the effects of genetic strain and feed restriction on the sexual and aggressive behaviour of male domestic fowl. Behaviour of 3 genetic strains (two broiler breeder strains and one commercial laying strain) on two feeding regimens (one restricted and one fed ad libitum) between 25 and 37 d of age was compared in this experiment. At 22 week of age, pairs of same-strain males were penned with 20 females of a broiler breeder strain. Behaviour was sampled from each pen for six 10-min periods during weeks 25, 27, 29, 33 and 37. Broiler breeder males displayed significantly more male-to-male and male-to-female aggression than laying strain males. Broiler breeder males chased females, forced more copulations and displayed little courtship behaviour compared with laying strain males. There were no behavioural differences between broiler breeder strains. It was expected that feed restriction would result in increased aggression. However, males fed ad libitum displayed the most male-to-male and male-to-female aggression. Sexual behaviour was not affected by feeding regimen. In conclusion, broiler breeder males display aberrant sexual behaviour and extremely high levels of aggression, which are associated with genetic differences, not feed restriction