White-footed mice, Peromyscus leucopus, were removed from a riverine peninsula to assess the effects of a river functioning as a barrier to movement. During an 8-mo period in 2001, 101 mice were live trapped and released across a river from the 14.2-ha peninsula. No small mammals were found to emigrate from the peninsula; 6 of the mice removed, however, exhibited homing behavior and immigrated across a fifth-order river and returned to or near the original site of capture. More studies at this temporal and spatial scale need to be conducted to demonstrate how natural barriers, particularly riverine peninsulas, could be utilized as experimental mesocosms in the study of small-mammal population dynamics at the landscape scale.