We investigated the effects of prior residence and previous cohabitation on the hierarchical relationships established between two wasps which were placed together in hitherto unknown surroundings. When two wasps which were both previously of alpha rank are brought together on the nest already occupied by one of them, the prior resident is likely to become the alpha individual in the newly established hierarchy. Prior residence is not a decisive factor however when two beta-ranked wasps come together. When two wasps which previously belonged to the same social hierarchy were separated for one week, during which time they both became alpha individuals before being reunited, they tended to revert to their original status. The low intensity level of their reactions suggests here that a persistent mnesic trace may have led them to adopt their previous respective attitudes.