Single females and pairs of Nicrophorus orbicollis Say were established as residents on carcasses to investigate interactions with intruders under natural conditions. When carcasses were exhumed 10-12 days later, one or more intruders were found on 19% of nests. Intruders expelled one or both of the residents and sometimes paired with a resident or another intruder of the opposite sex. An intruder tended to be larger than the same-sex adult it replaced on a carcass. A male intruder that joined a single female, on the other hand, was not necessarily larger than the resident female. There was indirect evidence that intruders committed infanticide and then started their own reproductive attempt on the carcass. Follow-up experiments in the laboratory indicated that interactions between intruders and residents were agonistic and that infanticide regularly occurred as a consequence of a takeover. Use of a genetic marker demonstrated that male and female intruders obtained reproductive benefits from infanticide. However, intruders sometimes cared for mixed broods consisting of their own young and young of a prior resident.