We characterized the intracellular symbiotic bacteria of the hematophagous glossiphoniid leeches Placobdelloides siamensis and a Parabdella sp. These leeches have a specialized structure called an "esophageal organ," the cells of which harbor bacterial symbionts. From the esophageal organ of each species, a 1.5-kb eubacterial 16S rRNA gene segment was amplified by PCR, cloned, and sequenced. Diagnostic PCR detected the symbiont in the esophageal organ and intestine. Phylogenetic analysis of the 16S rRNA gene(s) demonstrated that the symbionts from the leeches formed a monophyletic group in a well-defined clade containing endosymbiotic bacteria of plant sap-feeding insects in the gamma-subdivision of the Proteobacteria. The nucleotide compositions of the 16S rRNA gene from the leech symbionts were highly AT biased (53.7%).