The phylogenetic relationship of a nonflagellated, Gram-negative, rod-shaped intracellular bacterial parasite (BEV) of the leafhopper Euscelidius variegatus to other bacteria within the class Proteobacteria was determined by sequence analysis of 16S rDNAs. The presence of specific signature nucleotides showed this bacterium to be a member of the gamma-3 subdivision of the Proteobacteria. Phylogenetic analysis based on maximum parsimony placed BEV within a clade in the Enterobacteriaceae, which includes a number of bacteria that are facultative symbiotes of insects and have a common ancestor with Proteus vulgaris. Within this clade, BEV is most closely related to a bacterium identified as the secondary endosymbiote of another homopteran, the pea aphid, Acyrthosiphon pisum.