I present the first description of the nest, eggs, and nestlings of the Castelnau's Antshrike (Thamnophilus cryptoleucus). During June and July of 2003, I observed three nests of T. cryptoleucus in riverine habitats along the Río Amazonas, downstream from Iquitos, Departamento Loreto, Perú. The nests were deep, thin-walled cups suspended from the surrounding vegetation 1–3 m above the ground. The clutch size was two, and the eggs were cream-colored with reddish-brown markings. I found two nests on islands in the Río Amazonas and one nest on the mainland, providing evidence that T. cryptoleucus is not exclusively restricted to river islands. Two nests were in river-edge forest dominated by Cecropia, and one was in second-growth scrub next to a banana plantation, suggesting some tolerance of anthropogenic disturbance. All three nests were active during the period when the annual floodwaters were receding. Nest placement, structure of the nest, clutch size, and appearance of the eggs were all similar to those of the closely related Blackish-gray Antshrike (Thamnophilus nigrocinereus) and are typical of the genus.