ABSTRACT: The Global Stratotype Section and Point of the mid-Carboniferous boundary was selected in the Arrow Canyon section within the lower Bird Spring Formation at the first evolutionary appearance of the conodont Declinognathodus noduliferus (Ellison and Graves) sensu lato. The studied boundary beds, spanning a 38.8-m interval from the UpperMississippian into the Lower Pennsylvanian, occur in bryozoan-brachiopod-crinoidal grainstones/packstones with conodonts and foraminifers. The conodonts of the studied interval belong to the shallow-water Adetognathus-Rhachistognathus biofacies. The Adetognathus and Rhachistognathus species/subspecies A. lautus (=A. gigantus), A. spathus, R. minutus minutus, R. muricatus, R. primus, R. prolixus, R. websteri and transitional forms dominate the succession. Gnathodus girtyi simplex, G. lanei n. sp., G. defectus and transitional forms between them are abundant. The elements of the boundary-marker D. noduliferus s. l. and transitional forms between G. girtyi simplex and D. inaequalis are less common. The occurrences of Idiognathoides spp. are very rare. Neognathodus spp. only occur in the upper part of the studied interval. The mid- Carboniferous boundary stratotype has been studied by many workers but its conodonts have been rarely described systematically. The detailed updated descriptions of themost stratigraphically important taxa are given in this paper. The evolution of Gnathodus, Adetognathus and Rhachistognathus species within the mid–Carboniferous interval is discussed. Four lineages are summarized: 1) C. unicornis, A. unicornis, A. lautus and A. spathus; 2) G. girtyi girtyi, G. g. simplex and D. inaequalis; 3) G. lanei and G. defectus; and 4) R. prolixus, R. muricatus and R. websteri/R. primus/R. minutus.