Pro-group unethical behaviour (PGUB) refers to a set of behaviours that conflict with moral standards but are beneficial to the interests of an actor's workgroup. Drawing on social cognitive theory, this research investigates whether, how, and when perceived co-worker PGUB leads employees to engage in the same unethical behaviour. We propose that moral disengagement is a crucial mechanism through which PGUB is transmitted from co-workers to employees and that this relationship is further amplified by employees' group identification. The results of three field studies featuring different samples consistently highlight the mediating role of moral disengagement in the relationship between perceived co-worker PGUB and employee PGUB. Furthermore, employees who identify strongly with their workgroup are more likely to exhibit moral disengagement and learn their co-workers' PGUB than are employees with low group identification. The findings of this research provide insights for both theory and practice.