A house mouse population, homozygous for 8 pairs of Robertsonian metacentrics (2n = 24) has been discovered in the Po Valley area between the Ticino and the Adda. In the lower reaches of Ticino and along the Po specimens can be found which display hybrid chromosome sets between the Robertsonianly transformed karyotype and the Standard Mouse karyotype. East of the Adda river a house mouse population has been discovered with the 22-chromosome karyotype previously described for the Orobian and Valtellina populations. The relations of this new Robertsonian population with the other members of the Rhaetian system (Orobian, Poschiavo, Mesolecina and Engadine) are discussed.