Analysis of ancient human dental calculus for the presence of inclusions related to diet and dental health has been overlooked in anthropological literature. Small particles of archaeobotanical debris, which would otherwise not be preserved in the archaeological record, can become incorporated into unmineralised plaque on teeth during mastication and oral manipulation. When plaque mineralises into calculus, debris is preserved in situ. Samples of dental calculus (n = 18) were collected from the Danbury site (33OT16) in Ottawa County, Ohio and viewed under a scanning electron microscope for inclusions. Analysis yielded a variety of noticeable inclusions, including mineralised bacteria, calcium-phosphate crystalline structures and numerous phytoliths. Here we report the first evidence of fibres consistent with cotton (Gossypium spp.) embedded in the dental calculus from the Late Woodland component (900–1100 AD) of the Danbury site. Prehistoric cotton has not been previously documented in Ohio. The distinct morphology of the Danbury cotton and its presence in the Late Woodland component at Danbury suggests long-distance interaction at a time in Ohio when movement of exotic goods appeared to have diminished. These microscopic remains provide insight into paleoethnobotanical history of ancient Ohioans and attest to how analysis of dental calculus could be used to supplement other paleodietary and archaeological analyses. Copyright © 2010 John Wiley & Sons, Ltd.