The agonistic behavior of Exechesops leucopis (Jordan) (Anthribidae) that is associated with Styrax species (Styracaceae) is reported for the first time. This species shows remarkable sexual dimorphism in its cephalic morphology. The male head capsule is frontally flattened and laterally extended to form a pair of processes supporting eyes, whereas the female head capsule lacks processes. Head width exhibited a positive correlation with the body length. The head width was highly variable among male individuals and broadened with the increase of body length at a higher rate in males than in females. Males, particularly of large size, frequently exhibited territorial behavior on sprays bearing fruit or on individual fruit of Styrax trees where they copulated with females. Our observations revealed that males utilize their forehead as a weapon to exclude other males from their own territory, where they wait for females to mate and then guard the females after mating. Males that encountered obviously larger individuals fled without physical contact, suggesting that the male head width might be an indicator to assess the fighting ability of an opponent before actual fighting. The agonistic behavior was not observed between small males. They may adopt a mating strategy that is different from the strategy adopted by larger individuals.