With reference to social-ecological, self-determination, attributional, and social cognitive theories, the current study examined whether gender, age, altruistic motivation to defend victims, and tendency to blame the victims, at the individual level, and the prevalence of reinforcing and defending, at the classroom level, were associated with bullying. A sample of 901 Swedish students (9-13 years old, M = 11.00, SD = 0.83) from 43 classrooms filled out a questionnaire. Multilevel regression analyses revealed that the perpetration of bullying was positively associated with the prevalence of reinforcing at the classroom level and blaming the victims at the individual level, whereas it was negatively associated with altruistic motivation to defend victims of bullying at the individual level. Furthermore, students with high altruistic motivation to defend victims of bullying were less inclined to bully, independent of the classroom level of reinforcing. The current study suggests that bullying prevention and intervention programs should: explicitly target bystander behaviors, in particular to reduce the prevalence of reinforcing bullying; include efforts to strengthen altruistic self-concept and motivation to defend victims; and prevent, challenge, and counteract tendencies among students to blame the victim.