Males of some species of Agathidium Panzer have a prominent horn on the dorsal surface of the left mandible. The horn is unique in that it is dramatically asymmetrical unlike most male ornaments in insects. The horns can be homologized into four general morphological types. Scaling relationships are investigated for two species, A. angulare Mannerheim and A. pulchrum LeConte. These species exhibit a scaling relationship that is strongly discontinuous (sigmoid) with a prominent “switch point.” Measurement data (pronotal width vs. mandible height) are fit to a nonlinear regression model to determine the switch points and slopes at the switch points for the populations of each species. Behaviors associated with mating and other behaviors are described for A. pulchrum. Males use a head thrusting behavior to dislodge rival males from the substrate. Horned males appear better able to dislodge opponents. Head thrusting is also used less vigorously on females before and after mating.