Abstract: The endosymbiont of Acanthamoeba sp. CB/S1 was identified by orcein-carmine staining and 16S rDNA sequence analysis. The endosymbiont bacteria were rod-shaped and darkly stained, and irregularly localized within the cytoplasm. The length of the 16S rDNA was 1 534 bp and its DNA sequence was closely related to those of Candidatus Amoebophilus asiaticus and Acanthamoeba sp. KA/E21 with 98% homology. Phylogenetic analysis showed that the endosymbiont of CB/S1， the endosymbiont of KA/E21， Candidatus Amoebophilus asiaticus， the endosymbiont of Ixodes scapularis，and the endosymbiont of Encarsia pergandiella constitute a monophyletic lineage in phylogenetic tree.

Key words: Acanthamoeba, 16S rDNA, Endosymbiont, Candidatus Amoebophilus asiaticus