The stratigraphic significance of Cambrian and Ordovician acritarchs in Belgium and adjacent areas is discussed. It now appears that the age of the Deville group in the Ardenne (more precisely the upper part of this group) should be broadened to include not only the middle and late Lower Cambrian, but also the early Middle Cambrian. The Revin group comprises several well dated levels between the middle late Middle Cambrian to the latest Upper Cambrian. The lower part of the Salm Group (the so-called Sm1) comprises strata of early and late Tremadoc age. The age of the uppermost part of the Salm group (Sm3b) lies between the Llanvirn and the early Caradoc. Within the Brabant Massif, recent investigations of several boreholes have enabled the relative ages of the Oisquercq and the Mousty «assises» to be reconsidered, and the problem of their synchronism or diachronism to be addressed. If the fossiliferous samples prove to belong to these «assises», acritarchs would indicate a Lower or early Middle Cambrian age for the Oisquercq (comparable to the age of the Deville group) and an approximately Middle to Late Cambrian age for the Mousty (comparable to that of the Revin group). Comparison of the different areas studied shows a probable synchronism of the Deville-Revin boundary, the later possibly coincident with that between the Oisquercq and Mousty; a likely unconformity in the Revin group of the Rocroi Massif; sedimentary variations in the Cambrian (Mousty lacking in some places) and the Ordovician (Tribotte facies restricted to the Dyle area) of the Brabant Massif comparable with block faulting phenomena; existence of? eustatic late Tremadoc regression and contemporaneous unconformities; a possible confirmation, by means of bathymetric criteria, of a subdivision of Lower Palaeozoic strata into 3 megasequences, and a correlation of this cyclicity with that observed in southern Great Britain.