Burrowing behavior was assessed on 120 lab-reared house mice (Mus domesticus) derived from five geographic populations representing a north-south cline along the east coast of the United States. Mice were placed individually into Plexiglas containers filled with sand and peat moss, and their burrows were excavated 24 h later. Seven measures were taken and reduced by principal-components analysis to two factors for further analysis. Marked differences existed within, but not between populations, and members of full-sib families built qualitatively and quantitatively similar burrows. The lack of a geographic cline and the apparent high heritability of burrowing behavior do not lend support to its use as a major thermoregulatory adaptation.