A total of 3142 specimens of Pagurus comptus (White, 1847) were collected on the kelp beds in the Beagle Channel, Argentina, during 1999-2001. Fifteen of these hermit crabs were infested by a bopyrid provisionally identified as Anathelges cf. hyptius (Thompson, 1902). Of these parasites, 10 were attached to their hosts' pleons, whereas the remaining five were found inside the branchial chambers of the hermit crabs. All branchial parasites were immature females though one of them harboured a male. This is the first time that a postlarval stage of Anathelges is recorded inside the branchial chamber of its host. Whether this location is normal or erratic is unknown. This collection of Anathelges has made it possible for us to describe and illustrate four sequential female stages of this parasite, to extend the geographic range of distribution of this genus to the southern tip of South America, and to discuss the taxonomic value of some of the characters used to distinguish species in this genus.