Behavioural variation in primates has been well studied at the level of individuals by behavioural ecologists, and more recently at a population level by cultural biologists. Behavioural differences between groups of the same population, however, have rarely been considered. Here, I review intergroup variation in universally occurring behaviours between up to six neighbouring groups of vervet monkeys. Group differences have been found in several domains including foraging, conflicts, grooming and proximity. An exclusively ecological explanation for the observed variation appears implausible, as these monkey groups have overlapping home ranges, and their ecology is therefore very similar. The presence of a genetic bias underlying the observed group differences is also implausible, as males disperse between groups at sexual maturity and multiple times within their lives creating a continuous gene flow between the six groups. This leaves socially learned group-level traditions as the most plausible explanation. I discuss ways in which this working conclusion can be tested. The likely presence of group-level traditions in close proximity, well known in humans, has rarely been considered for non-human primates and adds an important dimension to research on conformity in non-human cultures.