Nymphalis antiopa (Mourning Cloak), a common daytime-flying butterfly, is known to exhibit territorial defense against small animals that share the same airspace. However, a bat seems to be an unlikely target for this aggression because bats are potential predators and diurnal bat activity is rarely observed. Here, we report territorial behavior of 2 Mourning Cloaks in response to a diurnally active Eptesicus fuscus (Big Brown Bat). This report is unusual in that it involves a non-predatory cross-phylum interaction, between an insect and a mammal.