Radiolarian assemblages in Holocene sediments from the Gulf of Mexico and Caribbean and the southern California borderland reflect overlying oceanographic conditions, such as general productivity, upwelling, and preservational parameters. A reconnaissance investigation of the lateral variations in the radiolarian assemblage with increasing water depth indicates that the changes are not consistent enough in either region for the establishment of a definite depth zonation; however a number of radiolarian species and higher taxa show some general trends with depth. Symbiotic taxa, and spumellaria in general, are more abundant in the thanatocoenosis of the Gulf region than off California and are indicative of oligotrophic conditions. Low oxygen content and high dissolved silica concentration in the bottom water of the Gulf of Mexico's Orca Basin lead to good preservation in this locality. Diversity, up to the family level, is greater in the California assemblages due to a mixing of radiolarian faunas and better preservation. There are more deep-water radiolarian taxa present at shallower depths, and all depths, in the California assemblages as compared to those of the Gulf due to more dynamic upwelling conditions which enhance these species both in the biocoenosis and thanatocoenosis.