Controlled laboratory experiments have delivered extensive and compelling evidence for the diffusion and maintenance of socially learned behavior in primates and other animals. Such evidence is rarer in the wild, but we show that a behavior seeded in a majority of individuals within vervet monkey (Chlorocebus pygerythus) groups may be sustained across several years. Here, we report results of two natural fission events in such groups that offer novel evidence of the resilience of socially transmitted group norms of behavior. Before fission, high ranked females exhibited an almost exclusive adherence to a group preference among two food options, originally introduced through a distasteful additive in one option, but no longer present in repeated later tests. Because of rank-dependent competition, low-ranked females ate more of the formerly distasteful food and so discovered it was now as palatable as the alternative. Despite this experience, low ranked females who formed the splinter groups then expressed a 100% bias for the preferred option of their original parent group, revealing these preferences to be resilient. We interpret this effect as conformity to either the preferences of high rankers or of a majority in the parent group, or both. However, given fissioned individuals' familiarity with their habitat and experimental options, we question the adequacy of the informational function usually ascribed to conformity and discuss alternatives under a concept of "social conformity".