During a radio-tracking study of North American beavers (Castor canadensis) in Arizona, we discovered three adult, lactating beavers using the same bank den at the same time. Two adult females 5.2 km downstream also were using the same den at the same time. For the first case, we documented use of communal dens by lactating adults on seven occasions during 68 days. For the second case, we documented communal use of a den on three occasions during 45 days. To test the hypothesis that these communal females were closely related, we used eight autosomal DNA microsatellites. Two females in the first den were first-order relatives. However, the other communal females were unrelated to the females with which they shared the den. We conclude that communal denning, where multiple females raise one or more litters in the same den, may have occurred in our study area and communal-denning partners are, in some cases, close kin.