This study investigated the prospective association between moral disengagement and bystander behaviors in bullying situations, including both defending and passive bystanding. A diverse sample of Canadian school children (N = 130; 68 boys and 62 girls; mean age = 11.36 years) participated in a three-wave longitudinal study over a 4-month period. Computer-based questionnaires were used to assess passive bystanding and defending by using both self-report and peer-report measures, as well as moral disengagement using self-report. Structural equation modeling was conducted to test longitudinal associations between moral disengagement and the bystander behaviors, resulting in an overall pattern that was consistent with sociocognitive theory of moral agency. Findings also revealed important sex differences and methodological issues, particularly with regard to the use of peer-nomination tools versus self-report tools.