The Simplon tunnel is a railway connection trough the Alps between Brig (Switzerland) and Iselle (Italy). Constructed at the beginning of the last century, it consists of two parallel, interconnected tunnels of 19.8 km each. Due to geothermal conditions, its temperature of 29°C is seasonally invariable. Stories about blind mice induced us to sample small mammals in the central part of the tunnel. We used 30 Longworth traps, set in 6 groups of 5 traps. After a prebaiting period of 2 weeks, the traps were opened during one night. We captured 10 Mus domesticus Rutty, 1772. A karyological analysis showed that they had the standard diploid number of 2n = 40, as mice from Brig. Mice from the Val d'Ossola (Italian side of the tunnel) had a karyotype of 2n = 24 with two specific Robertsonian fusion, Rb(5.8) and Rb(7.15). This "Domodossola race" belongs to the Lago Maggiore sub-groupe. As a conclusion, the tunnel colonisation took place from the north. With a density of about 5 - 10 mice per km, a rough estimate of the total tunnel population is about 200 - 400 mice. The few pick-nick left-overs from workers active in the tunnel cannot sustain such a population. It is concluded that the mice, as well as the regularly encountered Gryllus domesticus, are living from human faeces, dropped from the water closets of the trains. Low food resources, lack of predators and perhaps lack of accidents imply a density dependent population control, coupled with a low reproduction rate.