The California Current, the eastern limb of the North Pacific gyre, exhibits the following characteristics‘common to eastern boundary currents: wide, shallow, slow, diffuse boundaries, common upwelling, great seasonal variation, invasions of water masses from outside the system, and cold, low salinity waters. Studies on plankton tows and Holocene sediments have correlated components of the siliceous microplankton (radiolarians and some diatoms) with a number of those characteristics such as: the main directions of movement of the invading waters, the provenance of these waters, the presence and degree of upwelling, seasonality and its impact on the underlying sediments. Certain types of radiolarians have been found to be potentially useful in determining fossil anoxic and oxic conditions as well as paleodepth. This study involved analysis of box core sediment and plankton tow samples from the southern California continental borderland for radiolarian density, diversity, taxonomic makeup, and other features which were related to oceanographic and environmental conditions. Depositional environments were defined for the sediment samples and radiolarian indicators useful for paleoenvironmental interpretation were defined. A number of borderland environments were identified and the anoxic nearshore basin was found to have the best preservational qualities for radiolarians and thus the most representative radiolarian biocoenosis.