The habitat and nest sites of an island-endemic, the Másafuera Grey-flanked Cinclodes, are described for the first time. The species breeds only in natural cavities in steep rock-walls and rocky slopes or ridges. The nest entrance diameter ranges from 5.5 to 20 cm, the nest chamber being placed within the rock so as to be invisible from the outside. The nest sites found — solely on Alejandro Selkirk Island — were located from a little above the sea-level to the summit region, the preferred habitats being barren canyon bottoms with small streams, and humid alpine plateaus. Nesting extends from early December to late January, indicating a core breeding season from November to January. Fledgling numbers per clutch average about two.