The Cipero marl formation is defined by reference to a type section south of San Fernando, Trinidad, B.W.I. The priority of the name Cipero is indicated in a critical review of the published literature. Grounds for division into a lower (Paradise) and an upper (Cacatro) member are discussed. Criteria are listed for the recognition of three foraminiferal zones. Fossil evidence is given for believing that the period of deposition of the Cipero formation coincided fairly closely with the Oligocene epoch. The stratigraphic relations between the Cipero and other formations of the Naparima Basin are described. The probable milieu of deposition of the Cipero formation is discussed in the light of ecological evidence of its faunas and the geological history of the Naparima Basin. The conclusion is reached that the formation is an open-sea deposit of Globigerina-ooze type, mostly laid down at depths of 400-500 meters. In a regional sense the Cipero formation is shown to be a close equivalent in age and facies of the topmost Oceanic marls and younger marls in Barbados. Other Oligocene units described from the Antilles and Tropical America are better comparable with the Brasso and San Fernando formations of Trinidad, though correlation with the Cipero is possible on the basis of pelagic Foraminifera of limited life range.