Delimiting the spatial extension of a hybrid zone is essential to understand its historical origin and to identify the geographical and/or environmental factors which delimit it. Rhinella atacamensis and R. arunco are two sister species which together inhabit Chile between 25° and 38° S. Their distribution limits coincide at about 32° S, where recently it was reported that they hybridize in a small watershed (Pupío creek). Although the genetic evidence suggests that these two species form a hybrid swarm, they are not mixed homogeneously in the entire watershed, but rather are spatially segregated: R. arunco is found in the lower part of the creek and R. atacamensis in the higher part. An extensive exploration north and south of 32° S revealed other instances of hybridization, with the same pattern of spatial segregation within other watersheds. This study describes the hybrid zone combining mitochondrial sequences and nuclear AFLP markers. In the northern part, the hybrid zone is a narrow strip which crosses several watersheds and extends more than 130 km from NW-SE, so that R. atacamensis is found at higher altitudes towards the south. However, two points south of this strip show that the hybrid zone is more extensive and complex, and probably extends along the entire border of the mountain chains which form the watershed of the Aconcagua River (32°30' - 33° S). We propose an explanation for the origin of this hybrid zone considering paleoclimatic and orographic information, and briefly discuss the taxonomic implications of these results.