Predator recognition was studied in Dascyllus marginatus, a small Red Sea pomacentrid that forms mixed-size schools associated with branching corals of the genera Acropora and Stylophora. The reactions of variously sized D. marginatus to sudden exposure to a live predatory fish (Epinephelus fasciatus), a non-predatory fish (Acanthurus nigrofuscus), and an empty cell were monitored in the laboratory. Adult fish (49 to 60 mm) discriminated between predatory and non-predatory fish, whereas juveniles (13 to 17 mm) could not. The responses of single juveniles, pairs of juveniles, and juvenile/adult pairs to sudden exposure to a predator were compared. Juveniles paired with adults avoided the predator more than single or paired juveniles. A mechanism in the reef habitat of social facilitation of predator recognition and avoidance among mixed-sized school pomacentrids is suggested. The relevance of the findings of this study to the low survival of reef fishes shortly after settlement and their preference for coral heads inhabited by conspecifics is discussed.