ABSTRACT: Habitat degradation often results in resident displacement. One potentially important determinant of the lethality of such displacement is the nature of interactions between displaced individuals and the occupants of nearby, intact habitat. Here, we show that displaced individuals of the coral-dwelling fish Dascyllus aruanus have very low potential to join a new group due to targeted and cooperative aggression by conspecific resident group members. Residents were found to be relatively non-aggressive towards familiar displaced individuals seeking group re-entry but highly aggressive towards displaced unfamiliar prospective group members. The extent of individual resident aggression towards unfamiliar individuals correlated with group-based dominance rank, resident-to-intruder size similarity, and social group size, such that higher ranking resident group members disproportionately contributed to intruder defence. Subsequent group re-entry by displaced familiar individuals was observed 100% of the time, compared to an 18% group entry success rate for displaced unfamiliar individuals. Results indicate that pre-existing social bonds and associated cooperative aggression can generate greater habitat-disturbance lethality for displaced inhabitants than that predicted by mobility potential and alternative habitat availability alone. This implies that social group entry rules may be an important determinant of population resilience to patchy habitat disturbance.