The majority of polychaetous annelids are free living and the parasite occurrences are not very commom. Labrorostratus, a parasitic Arabellidae, occurs in the celoma of many polychaeta. The 33 individuals here examined were found in the celoma of one single specimen of Perinereis cultrifera, from the northern coast of São Paulo. As many as five developmental stages were distinguished, characterized by well difined differences, such as length and number of segments. Stage V corresponds to the most advanced, characterized by the presence of eyes and well developed mouth apparatus. The presence of a "stolon like" budding on Labrorostratus prolificus posterior end justifies the specific name.