Sexual dimorphism was found in the mandibles and other supportive structures used in male–male combat over females in Librodor japonicus. Observation of the behavior of male–male interactions of L. japonicus elucidated the influence of contestant size and resource ownership status on the outcome of male–male fighting. Interactions escalated in the presence of females, indicating the influence of resource value on the intensity of male fighting. At escalated fighting, the outcome of combat was influenced by size and ownership status. This study is the first on sexual dimorphism and male fighting in the Nitidulidae.