The results of the first comprehensive paleobotanical survey of the northern portion of the late Miocene Ogallala Group are discussed. Several species of fossilized fruiting structures from vascular plants (Berriochloa, Celtis, Cryptantha, Biorbia, Lithospermum, Eleofimbris, Carex, and Cyperocarpus) in the mid- to late-Miocene are reported. Fewer taxa (12 genera) were found in the South Dakota-northern Nebraska area than have been reported previously (20 genera) from southern Nebraska-Kansas strata in the same formations and approximately the same area of exposures. Sites of Barstovian age near the Kilgore Flora site indicate that grasses were commonly found in the region. The results presented here support the hypothesis that grassland savanna vegetation was widely distributed in the northern Great Plains by the Clarendonian age.