Whiteflies (superfamily Aleyrodoidea) contain eubacterial endosymbionts localized within host cells known as mycetocytes. Sequence analysis of the genes for the 16S rRNA of the endosymbionts of Bemisia tabaci, Siphoninus phillyreae, and Trialeurodes vaporariorum indicates that these organisms are closely related and constitute a distinct lineage within the gamma-subdivision of the Proteobacteria. The endosymbionts of whiteflies are unrelated to the endosymbionts of aphids and mealybugs. which are in two separate lineages.