Native grasses and forbs were once dominant in tallgrass prairies, but nonnative plants have largely replaced natives in most grassland of Illinois. Ample evidence indicates that white-tailed deer (Odocoileus virginianus) have a strong impact on native forests and prairie, but we know little about their impact on nonnative grasslands. This study tested the hypothesis that foraging by deer on native forbs and woody plants increases the dominance of introduced grasses as succession proceeds in old fields of central Illinois. We tested that hypothesis using fencing that excluded deer, but not other herbivores, from replicated plots in three old fields at different successional stages. The composition of the plant community changed rapidly in early succession and showed relatively little effect of deer, other than slowing the invasion by woody plants. In mid- and late successional fields, however, fenced plots had higher relative abundance of native forbs, particularly goldenrod (Solidago spp.), and lower relative abundance of introduced perennial grasses than did control plots. Thus, deer facilitated introduced perennial grasses and inhibited native forbs and woody plants in old fields, thereby delaying succession to deciduous forest.