This paper presents some observations recently made on three nests of the yellow-jacket, Vespula arenaria (Fabricius) [= diabolica H. de Saussure], which had been parasitized by Vespula adulterina var. arctica Rohwer, and on a nest of Vespula rufa var. vidua (H. de Saussure), which had been invaded by a queen of Vespula squamosa (Drury), suspected of being a facultative, temporary social parasite. To this are added some remarks on the origin and evolution of social parasitism in the genus Vespula.