Subsurface sedimentary successions in the Sosnowiec IG-1, Goczalkowice IG-1 and Potrojna IG-1 boreholes of Upper Silesia were studied for organic-walled microfossils in order to establish the biochronology of these otherwise sparsely fossiliferous strata. Taxonomically diverse associations of acritarchs allow recognition of the Lower, Middle and Upper Cambrian series and more detail ed zones within the Lower and Middle Cambrian. In relation to the Baltic craton, the area is a suspect terrane located within the Trans-European Suture Zone. The taphonomy of microfossils and the thermal maturation of organic matter, in the context of sedimentary structures and facies associations, indicate that the microfossils occur in situ in the depositional settings. The distribution of morphotypes and sizes of microfossils in various depositional environments suggest that there is no selective distribution of microplankton and thus no distinctive plankton communities occupying nearshore and offshore shallow-shelf environments. The biodiversity of Cambrian phytoplankton is reviewed in relation to global and regional geoevents. Severai major radiation and extinction events are recognized. The record from Upper Silesia is compared to the pattern of Cambrian secular biodiversity changes, revealing some regional variations at the Lower-Middle Cambrian boundary. The duration of the lowest Cambrian acritarch zones, the Asteridium tornatum - Comasphaeridium velvetum and Skiagia ornata - Fimbriaglomerella membranacea zones and their time-equivalent faunal zones of Platysolenites antiquissimus and Schmidtiellus mickwitzi, is estimated as 2-5 Ma each. The Upper Silesia terrane represents a distal portion of East Avalonia at the margin of Gondwana. The homogenous distribution of phytoplankton along the shelves of Baltica, Gondwana and Laurentia facing the Iapetus and Avalonian seaways indicates that these basins were still relatively narrow in Early Cambrian times, i.e. at the initial opening stages. The Upper Silesia terrane has been accreted to Baltica along the Krakow-Myszkow Fault Zone, considered to be an extension of the Tornquist Suture and possibly coinciding with the Trans-European Fault. Seventy-five form-taxa are described, with synonymies and a comprehensive compilation of their stratigraphic ranges and geographic distribution. The taxonomic status of a number of taxa is revised, rejecting so-called 'polymorphic genera' and arbitrarily chosen dimensional limits in the diagnoses. The new genus Duplisphaera is recognized, as well as eighteen new species: Adara undulata, Asteridium pilare, A. solidum, Celtiberium? papillatum, Comasphaeridium silesiense, Cymatiosphaera pusilla, Heliosphaeridium bellulum, H. exile, H. nodosum, H. serridentatum, Multiplicisphaeridium ramosum, M. sosnowiecense, M. varietatis, Solisphaeridium bimodulentum, S. cylindratum, S. elegans, Stelliferidium robustum, and Vogtlandia simplex. Six additional taxa are leEt under open nomenclature. The diagnoses of two genera, Revinotesta and Solisphaeridium, and three species, Solisphaeridium baltoscandium, S. flexipilosum, and S. multiflexipilosum, are emended. Nine new combinations of species are proposed: Duplisphaera luminosa, Heliosphaeridium lanceolatum, H. oligum, Lophosphaeridium latviense, Multiplicisphaeridum parvum, Polygonium varium, Revinotesta izhorica , R. saccata, and Solisphaeridium implicatum. The general taxonomic concepts of acanthomorphic and polygonomorphic acritarch genera are discussed in the view of recent evaluations.