The transgression of Oligocene continental deposits by the Pacific Ocean in latest Oligocene and earliest Miocene in central and southern California is represented in the rock record by widespread shallow marine deposits of the Vaqueros Formation. Fossils from 32 Vaqueros Formation localities in the northeastern Ventura Basin near Piru Creek have been studied and compared with specimens from previous paleontological studies. Previously unreported taxa from the Piru Creek area include Potamides sespeensis, Crepidula diminutiva, Sinum scopulosum, Ceratostoma sp., Ocenebra dorrancei, Rapana vaquerosensis, Olivella pedroana subpedroana, Anadara santana, Anadara santaclarana, Anadara hamelini, Ostrea eldridgei, Ostrea howelli, Anomia vaquerosensis, Eucrassatella granti, Lucina wattsi, Clementia pertenius conradiana, Chione cf. C. richthofeni, Macoma arctata, Solen gravidus baileyi, Panopea abrupta, and Balanus sp. Comparison of the megafauna of the Piru Creek Vaqueros with established Vaqueros biostratigraphy indicates that the Piru Creek beds are lower to middle Vaqueros age, which corresponds to the Zemorrian foraminiferal stage. No evidence was found for upper Vaqueros strata. In the Piru Creek area, the Vaqueros Formation ranges from 35 to 305 meters in thickness. This variation is due to truncation of some of the beds by the Agua Blanca thrust fault, as well as erosion of some strata prior to deposition of the overlying Modelo Formation. The Vaqueros Formation is divided into two informal members. The lower Member; from 35 to 95 meters thick, is predominantly a light brown fine arkose. The lower member interfingers with the underlying Sespe Formation. The upper Member; from zero to 230 meters thick, is predominantly a gray mudstone, with interbedded sandstone and siltstone lenses which are locally fossiliferous. The lower member contains a scattered megafauna. The gastropod Turritella inezana occurs in thin, highly indurated sandstone beds made up of up to fifty percent Turritella shells, along with a few shells of other species. Other taxa are found at various localities throughout much of the unit. The upper member is largely barren except for fine sandstone lenses which contain a diverse megafauna. Based on paleoecologic and sedimentologic evidence, the lower member is believed to have been deposited in a bay margin environment, and the upper member in a shallow shelf environment, transitional to deeper water.