Abstract
In this study, we report the second endoparasitic relationship for the oenonid Labrorostratus caribensis, previously described parasitizing a nereid polychaete. The new host is the syllid Haplosyllis rosenalessoae, in which it occupies 2/3 of its body cavity host. This is the second endoparasitic relationship between polychaetes and the first between both partners in known for Brazil, as well as the first record of the parasite outside the type locality (Caribbean) and of the host in the State of Bahia. We discuss on the interest of the finding of these king of parasitic relationships and provide an identification key to all knwon species of Labrorostratus.