Conodonts from U.S. Midcontinent cyclothems were studied for oxygen isotopes in order to constrain Pennsylvanian glacioeustatic sea-level fluctuations. Pennsylvanian deposits of the Midcontinent United States are composed of cyclic alternations of thin transgressive limestones, offshore gray to black phosphatic shales, and thick regressive limestones, a sequence that is underlain and overlain by nearshore to terrestrial shales with paleosols and coal beds. Glacioeustatic sea-level fluctuations are considered the primary cause for the formation of these cyclothems. Oxygen isotope analyses of conodont apatite from the black (20.1 ± 0.5‰, Vienna standard mean ocean water [VSMOW]) and gray shale units (20.5 ± 0.5‰, VSMOW) show lowest average δ18O values, whereas conodont elements from the regressive (21.0 ± 0.3‰, VSMOW) and transgressive limestone units (21.1 ± 0.6‰, VSMOW) are enriched in 18O. The maximum change in δ18O of conodonts from the black shale and carbonate units from individual cyclothems is 1.7‰. The 1.7‰ difference in δ18O compares relatively well to Pleistocene interglacial-glacial changes in δ18O of equatorial surface-dwelling foraminifers and suggests that Pennsylvanian glacioeustatic sea-level changes may have been of comparable amplitude. However, since the Pennsylvanian glacial maxima are represented by terrestrial sediments and are not documented in the conodont oxygen isotope record, Pennsylvanian glacioeustatic sea-level changes were probably larger than the 120 m fluctuations recorded for the Pleistocene glaciations.