The Changuinola Formation of northwestern Panama is described and formally defined. It consists of light-colored foraminiferal limestones and andesitic and dacitic volcanic flows and clastics. This formation is of Late Cretaceous age and includes the oldest dated sequence of sedimentary rocks known on the Atlantic coast of Panama. Deposition of the Changuinola limestones occurred in moderately deep to bathyal waters adjacent to an eastward extension of the "Guanarivas" volcanic island chain. A structurally more complex stratigraphic section along the Rio Lari in southeastern Costa Rica, about 75 kilometers northwest of the Changuinola type area, is also described. This second section contains a minor amount of carbonates lithologically similar to some of the Changuinola c rbonates, also some reworked Upper Cretaceous large Foraminifera in breccias. At one time, much of this Rio Lari section was thought to be Late Cretaceous in age, but more extensive sampling now seems to confirm a growing recognition of these Rio Lari beds as Paleocene to middle Eocene. Even so, the reworked Foraminifera point to the existence of nearby, but as yet unknown, Upper Cretaceous terranes that were subjected to erosion in Paleocene time.