We examined population dynamics of two species of small mammals, Mus musculus (house mouse) and Microtus pennsylvanicus, (meadow vole), in a highly disturbed dredge disposal site in eastern Virginia. Using mark-release-recapture methods, we trapped for 2 d three times per month for 13 mo, during which density of M. musculus declined from 104/ha to 37/ha and meadow vole density gradually increased from 8/ha to 41/ha. In all, 535 small mammals of seven species were trapped, with Mus constituting 65% and Microtus 28% of marked animals. Across the study, sex ratios were unity for house mice but nearly 1.5:1 favoring males for Microtus. Both species suspended breeding in winter, Microtus for slightly longer. Four habitat types on the 0.73-ha study grid were differentially used, less so by Mus than by Microtus. Populations of both species increased more through immigration than by breeding, and lifespans on the study grid were short, suggesting high levels of mobility for both populations. These attributes are discussed in the context of the dynamic environment in which these small mammals live.