A total of 357 house mice (Mus domesticus) from 83 localities uniformly distributed throughout Switzerland were screened for the presence of a homogenously staining region (HSR) on chromosome 1. Altogether 47 mice from 11 localities were HSR/ + or HSR/HSR. One sample of 11 individuals all had an HSR/HSR karyotype. Almost all mice with the variant were collected from the Rhone valley (HSR frequency: 61%) and Val Bregaglia (HSR frequency: 81%). For samples from most of thearea of Switzerland, the HSR was absent. There was no strong association between the geographic distribution of the HSR and the areas of occurrence of metacentrics. However, at Chiggiogna the HSR was found on Rb (1·3). Possible explanations for the HSR polymorphism are discusse
