The brown dog tick, Rhipicephalus sanguineus (Latreille 1806),was collected on dogs at the following localities in the province of Chubut, Argentina: Lago Puelo (42°04´S 71°38´W) and Puerto Madryn (42°46´S 65°02´W) during March 2001, and Sarmiento (45°35´S 69°04´W) during December 2000. These three localities represent contrasting environments, and the collections of R. sanguineus are the most southerly that have been reported to date. Rhipicephalus sanguineus is capable of completing its life cycle indoors, which may facilitate an extension of its range southward in South America. Amblyomma tigrinum Koch, 1844, a tick species adapted to different environmental conditions, was found on dogs in Puerto Madryn, but earlier collections of specimens that were probably this species (but that were identified as Amblyomma maculatum Koch 1844) have beenmade farther south.