Rhizocephala is a group of crustaceans that exclusively parasitizes other crustaceans. It is taxonomically placed within the class Cirripedia, the barnacles, with which it shares a unique larval type, the cyprid. The main objective of the cyprid is to find and irreversibly attach to a suitable substratum and initiate metamorphosis. In the presumed sister group to Rhizocephala, the true barnacles or Thoracica, metamorphosis leads to a juvenile filter-feeding version of the adult organism. In Rhizocephala the female cyprid settles on the integument of a crustacean and undergoes metamorphosis into a kentrogon that possesses a hollow cuticular-tube structure, the stylet, which penetrates the integument of the host and acts as a guide tube for the prospective internal parasite. The first, hitherto unknown endoparasitic stage of a rhizocephalan, the vermigon, was recently discovered (Glenner and Høeg [1995] Nature 377:147-150) and its migration through the hemolymph of the host, as well as its internal development, was described in Glenner et al. ([2000] Mar Biol 136:249-257). The present article provides detailed information on kentrogon and vermigon formation, the injection process, and the succeeding developmental stages up to the stage of the earliest primordium reported from the literature. The anlage of the ovary is traced back to the free-swimming cypris stage and it is implied that the mesoderm and ectoderm of the endoparasite are already differentiated in the cyprid.