Podisma pedestris exists as both an XO and neoXY form in the Southern French Alps. These chromosome types are shown to be contiguously allopatric and hybrid populations have been located in several places. Hybridisation appears to be occurring freely, but the hybrid zones are quite narrow. In several places the two distributions are separated by geographic barriers such as high mountain ridges and lowlands. The history of this XY chromosomal race is discussed in the light of the genetic dynamics of hybrid zones and the biogeography of this situation. Its origin is probably post-glacial, and it may be extending its range as a salient in one region. A comparison of the porperties of similar cases indicates that the formation of such chromosomal races may be a frequent event in relatively immobile species that are subjected to isolation and expansion by major climatic and ecological changes.