To characterize the biogeography of individual radiolarian taxa in relation to latitudinal climatic variation across the subtropical to subarctic zones, we investigated the spatial distribution of radiolarians in surface sediments along a 175°E traverse from 19°N to 48°N in the North Pacific Ocean. Census survey and statistical classification of taxa enabled us to recognize distinct radiolarian assemblages. Most characteristic taxa analyzed had restricted biogeographic ranges along the north–south traverse; the biogeographic ranges and abundance patterns related well to the modern oceanography of the upper layer. Water masses (subtropical, transitional, and subarctic zones) were characterized by abundance of certain taxa, and oceanic fronts (Kuroshio Extension, Kuroshio Bifurcation, and Subarctic Fronts) coincided with limits of species' biogeographic ranges and abrupt changes of frequency of some taxa. These results offer promise for pale-oceanographic interpretations of sedimentary records that will enhance the value of individual radiolarian species as keys to past water masses and hydrographic boundaries in the North Pacific.