About 2,450 carapaces, valves and fragments of ostracods have been extracted from 94 samples collected in the Fromelennes, Nismes and Moulin Liénaux Formations at Nismes, close to Frasnes (southern border of the Dinant Synclinorium, Belgium). Sixty-three taxa belonging to the Eifelian Mega-Assemblage are recognized. The only significant change as deduced from the ostracod fauna is the progressive transition from lagoonal and semi-restricted environments to open-marine environments in the upper part of the Fromelennes Fm. The Polyzygia beckmanni beckmanni Zone and the Favulella lecomptei Zone established on metacopid ostracods are recognized at Nismes, and the simultaneous occurrence of these two species in a sample collected in the upper part of the Sourd d’Ave Mbr implies the emendation of the definition of the P. beckmanni beckmanni Zone. The new definition is: presence of P. beckmanni beckmanni before the first occurrence of F. lecomptei . A new species, Ovatoquasillites nismesensis nov. sp., is described. The sedimentological analysis confirms that the transition of the Givetian and Frasnian stages does not correspond to a particular event. The evolution of the lithological curve across the Givetian / Frasnian boundary allows the recognition of 5 sequences recording a general drowning of the Givetian carbonate platform. No effective barrier system was active at that time suggesting that the Givetian carbonate platform was already dismantled prior to its definitive drowning.