This paper presents part of the data obtained during a multidisciplinary study of the Galapagos Rift in the western Panama Basin. It is based on photographic traverses made by the Angus towed camera system, observations made from the DSRV Alvin, and bathymetric maps constructed by the U.S. Navy using a multi-narrow-beam sonar system. The area studied is a 9-km section of the rift valley at 86¿W on the crest of a long and uncomplicated segment of the Galapagos Rift. In contrast to the terraced rift valley of the Mid-Atlantic Ridge, the Galapagos rift valley consists of a single trough, a few hundred meters deep, between faulted walls leading to low crestal ranges. The center of the rift valley contains a broad depression of sheet flows at the geometric axis and a lobate ridge of low volcanoes of pillow flows to the north. The volcanic ridge is highly fissured and contains several hydro-thermal springs on pillowed terrain adjacent to depressions containing young sheet flows. The youngest sheet flows may be less than 100 years old, while the entire age range of the young volcanic complex can be estimated at no more than a few thousand years. The young volcanic complex is bordered to the north and south by block-faulted marginal ridges that are significantly older. A model is proposed for the volcanic and structural evolution of the rift valley that begins with a zone of rapid extension, subsidence along boundary faults, and copious flows of sheet lavas from a shallow magma cmamber (0.5--1.0 km). After the crust has thickened, subsidences, extension, and flow rates decrease, and a ridge of small pillowed volcanoes is formed. A new axis of extension then develops to one side or the other of the volcanic ridge. This area begins to subside and fill with sheet flows. The older volcanic ridge is accreted to the nearest marginal high and straightened by normal faults. This complex then undergoes uplift and evidently become the crestal range, as the previous crestal range is rafted outward and gradually subsides.