Seven experiments were conducted to test the performance of adult deer mice (Peromyscus maniculatus bairdi) in discriminating and preferring siblings versus nonsiblings. In two experiments there was no indication of preferences based on odor alone. Further, there was no indication of kin discrimination in a seminatural enclosure. There was some indication that in a test apparatus, estrous females associate preferentially with male siblings rather than nonsiblings. The effect appears dependent on the presence of siblings during development. Although the finding of a preference for siblings in estrous females is counter to expectations from an inbreeding avoidance approach, this is one of a variety of findings to suggest that not all animals avoid inbreeding all of the time.