This paper provides a broad overview of spatial, architectural, and sensory relationships between rats and humans on British and American vessels from approximately the 1850s–1950s. Taking rats as my primary historical actors, I show how humans attempted to prevent the movement of these animals between ports across three periods. Firstly, the mid- to- late-nineteenth century, where few attempts were made to prevent rats from boarding ships, and where a multiplicity of human/rat relationships can be located. Secondly, the 1890s–1920s, in which port authorities erected anti-rat borders to lock these animals on land or at sea. Finally, the 1920s–50s, where ships were reconstructed to eliminate all possibilities of rodent inhabitation and to interrupt their transit between ports. Ship rats, I argue, not only demonstrate the fragility of historical rodent-control efforts, but also encourage oceanic historians to consider how animals have negotiated and shaped boundaries between spheres of land and sea.