The bess beetle, Odontotaenius disjunctus (Illiger), is a large subsocial beetle that inhabits decaying logs in which adults build tunnel systems and raise their larvae. Adults are territorial and both males and females protect tunnels from intruders. Though the behavior of many passalid beetle species, including O. disjunctus, has been studied, adult behavior toward conspecifics has not been thoroughly documented and many interactive behaviors have yet to be defined. We describe behaviors exhibited during laboratory interactions between non-cohabitating O. disjunctus adults. Twenty-four O. disjunctus were each tested in two experimental trials, once as a resident and once as an intruder, and in one control trial. We observed 18 behavior patterns and monitored them during trials. There were significant differences between control and experimental trials in some aggression behaviors, and in the total frequency of aggression, but no significant differences in submission, investigation, and neutral behaviors. Our data expand the behavioral repertoire for this species. Adults may use the described mild aggression behaviors in a territorial context, but we found no significant difference in total frequency of mild aggression between residents and intruders.