Epizoic diatom communities were studied on four turtle species (Chelonia mydas L., Eretmochelys imbricata L., Lepidochelys olivacea Eschscholtz and Dermochelys coriacea Vandelli) from the Eastern Caribbean, the Equatorial West Atlantic and the South Pacific. In the present study, we focused on one taxon of Navicula Bory de Saint-Vincent, described here as a new taxon, which was found on seven individuals of Dermochelys coriacea and one individual of Lepidochelys olivacea in French Guiana. The new Navicula taxon was found neither on Chelonia mydas (83 specimens examined) nor Eretmochelys imbricata (13 specimens examined). Furthermore, the new taxon appears to have a restricted biogeography, as so far it has only been reported from French Guiana. A species of Navicula is reported for the first time as a supposedly exclusive epizoic taxon. Navicula dermochelycola sp. nov. is characterized in detail ; its ultrastructure is described and compared with other members of Navicula.