Aggression is increasingly seen in most parts of South African society. Aggressive behaviour of boys in secondary school often results from frustrations caused by perceived high expectations of others regarding the role, locus of control, and personality of boys. Locus of control plays an important role in a person's perception concerning a situation and possible reactions to what is happening, or should be happening. A 56-item questionnaire, based on Rotter's "Locus of control" questionnaire, and the DIAS Scale were used. The questionnaire was completed by 440 boys in Grades 9, 10, and 11. Various factor, item and differential statistical analyses were conducted. Three constructs were identified, i.e. physical, verbal, and indirect aggression. Results indicated that locus of control has a significant influence on verbal and indirect aggression. The differential analysis indicated that contextual variables (language of tuition, age, and grade) play a significant, but not substantial, role in aggression. Furthermore, boys with an internal locus of control are significantly and substantially less aggressive than boys with an external locus of control, with respect to physical, verbal and indirect aggression.