ABSTRACT: Biostratigraphy of the Monte Alegre, Itaituba, and Nova Olinda formations (TapajÃ³s Group) in the Amazonas Basin is updated based on conodonts obtained largely from subsurface core samples. Four conodont zones are defined: the upper Bashkirian Neognathodus symmetricus and Diplognathodus coloradoensis zones, and the lower Moscovian D. ellesmerensis and Idiognathodus itaitubensis zones. They are correlated with Morrowanâ€“Desmoinesian foraminiferal occurrences and with Langsettianâ€“Asturian microfloras described previously. Earlyâ€“Middle Pennsylvanian conodont faunas are diverse and abundant. They are compatible with the location of theAmazonas Basin in low latitudes at that time. Subsequent climatic change to an arid, high salinity basin, and, later, subaerial conditions, restricted the conodont occurrences in upper Moscovian strata. A few middle-late Desmoinesian, later Pennsylvanian, and probable Permian faunas were recovered for the first time from beds with Gzhelian-Cisuralian microflora.