Two species of rails, Porzana rua n. sp. and Gallirallus ripleyi n. sp., are described from bones of late Holocene age found in caves on Mangaia, southern Cook Islands. Their relatively small pectoral elements show that both of these species were flightless. Porzana rua resembles most closely the living P. atra of Henderson Island and the recently extinct P. monasa of Kosrae Island, Carolines. Gallirallus ripleyi is most similar to the recently extinct G. wakensis of Wake Island. Some combination of predation and habitat alteration by humans and introduced mammals (rats, dogs, and pigs) is probably responsible for the extinction of P. rua and G. ripleyi within the past 1000 years. Fossils of a third species of rail from the Mangaian caves are referred to the living species Porzana tabuensis, although these specimens may represent an undescribed subspecies. Porzana tabuensis might survive on Mangaia and elsewhere in the southern Cook Islands, although entire specimens have never been collected. An X ray of the only two specimens (skins) of Porzana monasa (Kittlitz) shows that this species from Kosrae (Kusai) Island, Carolines, was flightless or nearly so. It is likely that all islands in the Pacific were inhabited by one or more species of flightless rail before the arrival of humans. In both Porzana and Gallirallus, at least one early wave of colonization produced flightless species throughout Oceania, followed by a less thorough and much more recent (probably late Holocene) wave of colonization by the volant P. tabuensis and G. philippensis.