Mexican jays live in stable social groups of 5-25 individuals. Groups show complex social organization, with dominance hierarchies and defence of territories against nonmembers. We tested whether Mexican jays displayed vocal recognition of group membership in a population of colour-banded birds in southeastern Arizona, U.S.A. We conducted playback trials of primary calls recorded either from an identified member of the target group or from a member of a nearby but noncontiguous group. Calls were played near established feeding stations during regular feeding sessions. The jays responded differently to calls from outsiders and insiders, with more calling, faster approach latency and more entries into a response circle (i.e. within 5m of the speaker) in response to calls from outsiders. Thus, the primary calls of jays are used to recognize group membership. Mechanisms by which this recognition is mediated are discussed.