Abstract: Vorticellids are eukaryotic organisms belonging to the order of the protozoan ciliates Peritrichida. It is a genus of a bell-shaped or cylindrical organisms that could live as free-living organisms or as parasites. They are present especially on submerged substrates, but can be found on the leaves and roots of aquatic plants, on filamentous algae. They have been also described attached to amphibians, fishes and gastropods. Theses parasites are present also in the gills of fishes or shrimps and very few studies described these parasites on crustacean gills. The objective of this study was to characterize the parasites found attached to the gills of Uca rapax living in the mud of mangroves in various sites in Guadeloupe. Material and methods Uca rapax individuals were manually caught in a mangrove area called "Manche à eau". After unhooking these parasites from the gills, two experiments were done: - Ultrastructural analysis by Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). - Genomic analysis by extraction of the whole DNA, PCR amplification of 18S rDNA using the universal primer set (1F-5R) before cloning with the pGEM®-T Easy Vector Systems kit and sequencing. Results SEM analysis reveals the presence of a bell-shaped parasites containing sties on their surface suggesting that they are vorticelids. No bacteria were observed on the bell but the stalks were covered by various morphotypes of bacteria. According to TEM analysis, many digestive vacuoles containing a lot of bacteria were observed. Phylogenetic analysis of the 18S rDNA sequences confirm that such parasites belong to Vorticella. Conclusion The results presented here show a high presence of vorticelids colonizing the gills of the crab Uca rapax. We allow for supposition that the proliferation of vorticelids on the gills is based on the bacterial epibionts colonizing the surface of gill filaments in this brachuryan species. Indeed, vorticella feeds on these symbiotic bacteria recently described from gills of crustaceans of the families Sesarmidae; Gecarcinidae and Ucididae. On the other hand, no bacteria and consequently, no vorticelids were detected in individuals of the Coenobitidae family (hermit crabs).