Burrow systems constructed by house mice (Mus musculus domesticus) in four large outdoor enclosures were excavated. Burrows ranged from 10–835 cm in length, contained 1–7 entrance holes and had 1–8 distinct tunnel segments and 0–5 cavities. Longer burrows had more segments, more entrances and at least one cavity. Only a few nest cavities and burrow segments contained stored food, probably because we excavated burrows in summer when food was abundant. We also recorded a novel behavior we termed ‘burrow cleaning’. During March and April we found many burrows with enlarged holes and considerable debris and/or grass deposited at burrow entrances. We propose that this burrow-cleaning behavior is designed to remove winter accumulations of older decayed material that may contain parasites. Female house mice occupy burrow systems that are longer, contain more entrances and more branches and have more cavities than burrow systems occupied by males. Possible functions of the various burrow systems include sites for nesting females and their litters, locations for communal nests during winter months and places where male mice can live in the summer months.