The pelagic lithofacies of the Mead Hill Formation, eastern Marlborough, New Zealand, contain the most complete record known of radiolarian evolution through the Cretaceous-Tertiary (K-T) transition. The K-T boundary is well-defined, both by microfossils and the "impact" signal of the boundary clay, and the earliest Paleocene record is relatively intact within a Late Cretaceous to Late Paleocene sequence of limestone, porcellanite, chert, and marl. This monograph presents systematic descriptions of 98 radiolarian species or species groups, within 58 genera, which comprise the main elements in the radiolarian faunas of four Cretaceous-Paleocene sections in eastern Marlborough: Woodside Creek, Wharanui Point, Chancet Rocks, and Flaxbourne River. The distribution of these species is documented in secondary localities in inland Marlborough (Mead Stream), Wairarapa, Chatham Island, Lord Howe Rise (DSDP Site 208), and Campbell Plateau (DSDP Site 275). Six new Cretaceous-Paleocene species are described: Protoxiphotractus wilsoni, Lithomelissa? aitai, Botryostrobus? parsonsae, Phormostichoartus strongi, Eusyringium woodsidensis, Lithocampe wharanui. Four new Paleocene species are described: Haliomma teuria, Lithelius marshalli, Clathrocyclas australis, Buryella kaikoura. Improved knowledge on the stratigraphic distribution of the species treated, as well as improved foraminiferal age control at Flaxbourne River, are incorporated into revisions to previous radiolarian zonations for the New Zealand region. No evidence is found for mass extinction of radiolarians across the Cretaceous-Tertiary (K-T) boundary in Marlborough. Estimates of species extinctions range from 0-8% of the total fauna. No species have last occurrences directly below the boundary, but five species are either restricted to the Cretaceous or have only isolated occurrences in the Paleocene, possibly due to reworking. Most Cretaceous survivors disappear in a relatively short interval in mid Early Paleocene (63.5-63 Ma) following a period of high siliceous plankton productivity. It is within this earliest Paleocene interval (65-63.5 Ma) that 60% of the recorded Paleocene species first appear, including the ancestors of several important early Tertiary lineages: Amphymenium splendiarmatum, Lithelius s.s., Amphicraspedum prolixum gr., Dorcadospyris, Dictyophimus, and Buryella.