We studied the poorly understood Black Catbird (Melanoptila glabrirostris), a near threatened mimid, at Lighthouse Reef in northeastern Belize. A resident of coastal lowlands and offshore islands, this endemic species of the Yucatan Peninsula has been reported as extirpated from several localities and has declined in numbers at other sites. We found it relatively common on the larger of two islands that comprise Northern Two Cayes from 18 to 25 June 2005. It had not been reported there since first discovered at Lighthouse Reef in 1862 and was considered extirpated until we rediscovered it. The Black Catbird at Northern Two Cayes displayed fierce intraspecific territoriality and both males and females defended against aggressors. However, it exhibited no interspecific territoriality toward its nearest avian associate, the Mangrove Warbler (Dendroica petechia bryanti). It used wing-flashing in territorial defense, mating rituals, and while foraging on the ground. We estimated ∼10 pairs of Black Catbirds in a 3-ha study area in the buttonwood-coconut (Conocarpus-Cocos) ecosystem but made no attempt to estimate the size of an apparently larger population in the more extensive area of coastal scrub on the remainder of the island. The defended territory of the pair studied most extensively was 100 × 25 m, centering on a buttonwood (Conocarpus erectus) grove and included numerous coconut (Cocos nucifera) trees.