Cyclothems in the Dunkard group (uppermost Pennsylvanian and/or lower Permian) m Pennsylvania, West Virginia, and Ohio comprise an alternating sequence of alluvial-plain and lacrustrine delta-plain sediments and show an asymmetric sequence of depositional environments imposed on symmetrical cycles of clastic gram size and of submergence. The widespread development of cyclothemic deposits in other late Paleozoic basins as well as in the Dunkard requires explanation. The synchroneity of cyclothem development throughout wide areas as well as other cyclothem characteristics prevents acceptance of any theories of local control; theories of regional diastrophism involve unknown tectonic mechanisms and do not explain certain other patterns of cyclothem development and occurrence; glacial-eustatic theories require repeated formation, as yet unverified, of continental glaciers and involve subsidiary assumptions about climatic fluctuations. Some characteristics of Dunkard (and possibly other) cyclothems, particularly the evidence for concurrent channel cutting and interchannel deposition, suggest that the increased competence of rivers resulted from changes in discharge rather than from modifications of regional slope by uplift or by lowered sea level. Climatic cycles would cause such changes in stream runoff, would account for the principal features of cyclothems, and are known to have affected late Cenozoic erosion and deposition.