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A Silurian radiolarian fauna based on 18 species is described from cherts from three sections in two quarries in the Sepon Mine area, central Laos. Each section contains distinctive assemblages butFutobari morishitai,F. solidus,Zadrappolus yoshikiensisandZ. tenuisare found in all three sections and are characteristic of the Ludlow–PridoliZ. yoshikiensisAssemblage. In addition, the Hoei Yang pit section containsZadrappolus hitoeganensis,Pseudospongoprunum sagittatum,Futobarisp. andSecuicollactasp. Section CQ containsPseudospongoprunum sagittatum,P. tazukawaensis,Praespongocoelia parva,P. sp.,Rotasphaerasp. cf.R. quadrata,Secuicollactasp. cf.cassa. Section QM also containsPseudospongoprunum parvispina,P.sp.,Zadrappolussp. andDevoniglansus(?)seponensisnew species Thassanapak is common. Evidence from Silurian and Devonian strata of the southern Truong Son Terrane indicates deepening towards the southern margin as defined by the Thakhek–Danang Shear Zone and exemplified by deep-marine Ludlow–Pridoli radiolarian cherts in the south, shallowing to fossiliferous, shallow-water sediments to widespread Old Red Sandstone type terrestrial facies 85 km further NE. The radiolarian cherts are in faulted contact with probable Llandovery limestone and Upper Ordovician – Llandovery graptolitic shales. The graptolitic shales are overlain with slight angular unconformity by the submarine fan, volcaniclastic-dominated, Namphuc Formation of Llandovery age. This fan received andesitic materials from the nearby Long Dai Volcanic Arc which extends eastwards to the Tamky Suture in Vietnam. This plutonic–volcanic arc was probably maintained by northwards subduction along the trend of the Permian–Recent Thakhek–Danang Shear Zone.