Blastocrithidia culicis and Crithidia deanei are trypanosomatids that harbor an endosymbiotic bacterium in their cytoplasm. In prokaryotes, numerous proteins are essential for cell division, such as FtsZ, which is encoded by filament-forming temperature-sensitive (fts) genes. FtsZ is the prokaryotic homolog of eukaryotic tubulin and is present in bacteria and archaea, and has also been identified in mitochondria and chloroplasts. FtsZ plays a key role in the initiation of cytokinesis. It self-assembles into the Z ring, which establishes the division plane during septation. In this study, immunoblotting analysis using a FtsZ polyclonal antibody, revealed a 40-kDa band characteristic of FtsZ in endosymbiont fractions and in whole trypanosomatid homogenates, but not in whole cell extracts of aposymbiotic strains. Confocal microscopy and ultrastructural analysis revealed a specific and dispersed labeling over the endosymbiont. Bars and ring-like structures, which are suggestive of the presence of Z-rings, were never observed, even during the division of the symbiont. This peculiar distribution of FtsZ may represent an arrangement of cytoskeleton protein intermediate between prokaryotic and eukaryotic cells. The endosymbiont ftsz gene was completely sequenced after amplification of DNA from symbiont-bearing trypanosomatids or from pure endosymbiont fractions, using PCR and specific primers. The sequences obtained from the endosymbionts from C. deanei and B. culicis were very similar, and were most closely related to bacteria from the genus Pseudomonas.