ABSTRACT: Conodont associations found in a carbonate succession cropping out in the Sierra AguaVerde (central Sonora, northwestern Mexico) indicate that the studied section spans the mid-Carboniferous Boundary. Declinognathodus noduliferus s.l., the index conodont for the base of the Pennsylvanian, is here reported for the first time in the studied area. The conodont fauna found in the lower part of the section is dominated by Taphrognathus varians, Gnathodus texanus, and Bispathodus n. sp. A., and also contains shallow-water conodonts such as Cloghergnathus sp., Hindeodus cristula, Cavusgnathus unicornis, Rhachistognathus prolixus, and Rhachistognathus muricatus. Pennsylvanian conodont fauna contains both cavusgnathids and gnathodonthids, including Adetognathus spathus, Adetognathus lautus, Adetognathus inflexus, and species of the Declinognathodus noduliferus group. Conodont associations found in Sonora are similar to those reported in other areas of North America, indicating that the shallow water rocks exposed in northwestern Mexico were part of a carbonate shelf connected to the epicontinental sea that covered most of the Laurentian craton during late Paleozoic times. In addition, some aspects on mid-Carboniferous conodont systematics and biofacies also are reviewed herein.