Two species of Peromyscus, the woodland deer mouse (P. maniculatus gracilis) and the white-footed mouse (P. leucopus), are found together in the forests of the northern Lower Peninsula of Michigan. Deer mice have become rare and their rate of decline appears to be accelerating. The relative abundance of gracilis in this area, measured as % gracilis in collections of Peromyscus, has declined from over 40% before 1931 to around 6% in 2003, and in detailed recent studies at one site, from over 50% of Peromyscus captures before 1996 to less than 10% in 2003. Numbers of lecucopus at the University of Michigan Biological Station, northern Lower Peninsula, are related to the length of the winter (measured as the date of ice break-up); it appears that few leucopus survive long winters (when ice breaks up in late April or early May). We suggest that a recent tendency for winters to end early (revealed for this area by 100+ years of ice break-up records for Grand Traverse Bay, Lake Michigan) may be responsible for the deline in gracilis populations.