Neogene radiolarian assemblages were examined from California, the offshore waters, and adjacent areas. Three time plans, 10, 8, and 5 Ma, were selected for study and were determined by the morphologic bottom of Diartus hughesi, the morphologic top of D. hughesi, and the morphologic bottom to acme of Lamprocyrtis heteroporos respectively. The general, symbiotic, warm water sphere, and cold water sphere diversity and abundance parameters of the radiolarian populations were plotted. The dominant trends in these parameters, at all three time planes, were primarily in the east-west (cross-current) direction. Secondary trends in the north-south (along current strike) direction were also exhibited. The radiolarian assemblage parameters offer a means to investigate in more detail the complex oceanographic history which existed during the time that the thick and widespread upper siliceous facies of the Monterey Formation was deposited. In this study two of the most significant oceanographic features reflected by these parameters were the California Current width and the degree of oceanographic convergence development off Baja California. The California Current appears to have been narrower (closer to shore) at 8 and 10 Ma while the convergence off Baja appears better developed at 5 and 10 Ma.