Study of more than 1350 sediment samples from three bays facing the Pacific coast of Baja California shows that the sediments are closely related to oceanographic conditions. At Sebastian Viscaino Bay the sediments are coarsest and of highest calcium carbonate content in a strait that is swept by tidal currents. Adjoining bands of progressively finer sediment result from lower velocities of the water at both ends of the strait. At one end of San Cristobal Bay the sediments are coarse and of high calcium carbonate content owing probably to local upwelling of water. The rest of the bay is an area of slow deposition of detrital sediments and thus it contains the authigenic minerals glauconite and phosphorite. The third bay, Todos Santos, is largely closed off from the open sea by protecting islands; accordingly, its sediments show a decrease in grain size from shore to deep water. Sediments of the same grain size in the different bays do not necessarily have the same percentage of calcium carbonate or of organic matter because of the different environmental controls. Such present-day background knowledge is useful in interpreting ancient sediments with respect to depositional environments.