Castelnau’s Antshrike is a large black Thamnophilus antshrike that is restricted to the floodplain of the Amazon River and its larger tributaries in western Amazonia. It primarily occupies taller vegetation on islands in these rivers, although locally it also occurs in similar habitats on the adjacent "mainland". The preferred habitat of Castelnau's Antshrike is the midstory of taller forest on larger, more established islands, as well as in dense forest with an understory of Heliconia on older islands; less frequently, it occurs away from islands in seasonally flooded forest. Males are entirely black with extensive white fringing to all of the wing coverts. Females are unique in being entirely black, but lack the white fringing on the wing coverts. Both sexes have a semiconcealed white interscapular patch, which they flare when agitated. Castelnau's Antshrike can be distinguished from similar sympatric species, such as the White-shouldered Antshrike (Thamnophilus aethiops), by its preference for successional habitats, rather than the interior of lowland evergreen forest. The primary song of Castelnau's Antshrike is a bouncing ball ukk-ukk-ukk-ukukukukuk, speeding up at the end. As in other species of Thamnophilus antshrikes, both sexes call back and forth, and characteristically pump their tails when vocalizing. Castelnau's Antshrike occurs in isolated pairs and does not join mixed-species flocks, which are not prevalent on river islands.