ABSTRACT: Coarse size (>63Âµm) radiolarian fl uxes along the Equator in the central and western Pacifi c between 145°E and 160°W (Stations MT3, MT4, MT5, MT6 and MT7) were examined during January 1999 to January 2003 in order to evaluate how radiolarian assemblages respond to the El NiÃ±o Southern Oscillations (ENSO). Radiolarian fl uxes were constrained by the water masses: the Western Pacifi c Warm Pool (WPWP) located in the west and the Equatorial Upwelling Region (EUR) east of the WPWP in the central equatorial Pacifi c. In general, radiolarian fl uxes gradually increased from the western sites to the eastern sites, responding to the increases in nutrient supply and primary production in the euphotic layer. Notable annual cycles of radiolarian fl uxes observed at the two Stations with nearly complete time series (MT3 and MT5) were likely corresponding to the rainy seasons of Papua New Guinea (MT3) and the extent of upwelling intensity in the equatorial Pacifi c (MT3 and MT5). With transition from La NiÃ±a to El NiÃ±o during 2001 to 2002, high radiolarian fl uxes were coincident with cooler surface water. Among the radiolarian taxa, Lithomelissa sp. group and Pseudocubus obeliscus Haeckel 1887, which were mainly dwelling in the surface water in the EUR, responded distinctively to the WPWP excursion and decreased their fl uxes as surface water temperature rose.