Reinvestigation of the Thyle river section, in the southern part of the Brabant Massif, has led to a better understanding of acritarch distribution in the Middle Ordovician Abbaye de Villers and Tribotte formations, and their stratigraphical significance in the type localities of both formations. The middle and upper parts of the Abbaye de Villers Formation and the lower part of the Tribotte Formation yielded an acritarch assemblage corresponding to the Frankea hamata-Striatotheca rarirrugulata Acritarch Zone of the English Lake District, demonstrating a late Arenig (late Fennian, early Darriwilian) age. Two species, Adorfia firma and Arbusculidium filamentosum, not recorded in the overlying Rigenée Formation, probably have their last appearances below the Arenig-Llanvirn boundary. Acritarch (late Arenig, late Fennian) and chitinozoan datings (middle Arenig, Whitlandian pro parte) of the middle part of the Abbaye de Villers Formation suggest correlation with the older part of the Frankea hamata-Striatotheca rarirrugulata Acritarch Zone. The uppermost part of the Tribotte Formation shows palynological similarities with the overlying Rigenée Formation. The exact position of the Arenig-Llanvirn boundary in the Thyle river section is not yet known and does not necessarily coincide with the Tribotte-Rigenée lithostratigraphical boundary.