Marsh rice rats (Oryzomys palustris) inhabit wetlands and occasionally occupy adjacent uplands. I hypothesized that demographic factors, environmental factors, and/or prey availability influenced their use of upland habitats. Rice rats were monitored on livetrap grids that extended from low marsh through upland habitats near Galveston, Texas. Diet was determined from fecal samples, and regression models evaluated influence of environmental factors on upland use. Density was highest in wetlands during summer and autumn and highest in uplands during winter and spring when tides were high and temperatures low. Adults were more common in wetlands, and juveniles were more frequent in uplands, whereas abundance of subadults was similar in wetlands and uplands. Rice rats ate primarily aquatic organisms and wetland vegetation in both habitats; upland plants were a minor component of the diet of rice rats in uplands. I conclude that uplands were not primary foraging areas but served as sink habitats for dispersers and as refuges during high tides. Given the highly fragmented nature of these wetlands, uplands may have significant conservation value as sinks in a metapopulation. These results support views that current laws for wetland delineation are ecologically incomplete because uplands are not protected and that buffer habitats or transitional areas adjacent to wetlands should be included in legislation and management plans.