From March to October 1986, 35 house mice, Mus domesticus, were fitted with radio-transmitters and tracked in and around a Colorado feed shed for an average of 5 (range 2 to 17) days. 5 of 36 simultaneously tracked pairs of lactating females used identical nesting sites. Genotypic similarity at 5 enzyme loci, assayed from biopsies of blood and toes, indicated that these females were more genetically alike than would be expected if pairing occurred randomly among females. We posit that female house mice recognize and preferentially form communal nests with close relatives. Aside from the communally nesting females, minimal overlap occurred among home ranges of lactating females. Only 4 of 10 adult male home ranges appreciably overlapped female home ranges. The home ranges of these 4 males overlapped less than the other adult male home ranges suggesting that males defend one or more females for access to mating. The possibility that communally nesting females are nursing each other's pups is discussed.