Neighbor-stranger discrimination is the ability to recognize and respond differentially to familiar and unfamiliar conspecifics. This type of recognition is common in species that aggressively defend territories, such as the colonially nesting Great Black-backed Gull (Larus marinus). This species has a large agonistic signal repertoire, which includes the “yeow,” a short-range threat vocalization. A playback experiment was used to determine whether Great Black-backed Gulls are capable of recognition on the basis of this call type. Subjects maintained alert postures for significantly longer in response to unfamiliar yeows than to those of their mates or neighbors. To my knowledge, this is the first study to demonstrate neighbor-stranger discrimination on the basis of a threat vocalization.