We hypothesized that the ability of the semiaquatic rice rat (Oryzomys palustris) to colonize over water is responsible for its ubiquity on the frequently overwashed Virginia barrier islands. A mark and recapture program was conducted from 20 May to 24 August 1989 on three adjacent islands to determine the frequency, direction and causes of the movements. During this study, 11 of 166 island residents migrated for the duration of the study to other islands. All recorded crossings were made from a small island to larger islands. The demographic composition of the overwater dispersers was a statistically random sample with respect to age and sex from the resident population, unlike rice rat overland dispersers that were predominantly juveniles.