Vigilance activity is usually considered an adaptive advantage of group living conferring protection against predators. An individual of a prey species is assumed to benefit from an increase in the number of group members by reducing its own investment in vigilance. However, the proximate mechanism generating the decrease in vigilance with increasing group size is unclear. Most models of this relation assume that either individuals coordinate their scans in nonoverlapping bouts of vigilance or animals scan independently of one another.We studied the relation between vigilance and group size in the eastern grey kangaroo, Macropus giganteus, analysing vigilance at two structural levels, individual vigilance and collective vigilance of the group. We tested both assumptions, coordination and independence of scanning. The time that an individual spent vigilant decreased with group size. However, the time during which at least one individual in the group scanned the environment (collective vigilance) increased up to a group size of seven individuals but decreased thereafter. Analyses revealed that individuals neither coordinated their scanning in an asynchronous way nor scanned independently of one another. On the contrary, both the onset and the end of individuals’ scanning bouts were synchronized between group members, producing waves of collective vigilance. Bouts of vigilance and nonvigilance activity tended to be synchronized. Our results appear to support a hypothesis of allelomimetic vigilance