Abstract Karyotypic variation due to the centric (Robertsonian: Rb) fusion of chromosomes is a widespread phenomenon among small mammal species. In 1993, we described a house mouse chromosomal hybrid zone in Upper Valtellina (SO). Here, we found mice with 32 different karyotypes, including the standard, or all-acrocentric race (2n=40), four Rb races (2n=22-26) and 27 hybrid types (2n=23-39). This hybrid zone presents a unique opportunity to study the role of Rb fusions and races in speciation. We have been studying this dynamic hybrid system using a wide variety of techniques: karyology, histology, breeding, mark-recapture and DNA sequencing. All four Rb races appear to be closely related, but 40AA has probably been introduced recently into the valley. However, the fertility of laboratory-reared hybrids between several of these races (24UV, 26POS, 40AA) are lower than expected compared to homozygotes and previous studies. Effective subpopulation size and migration rates within and between villages are also relatively low. We discuss the use of these parameters to study the process of speciation in ongoing computer simulations.