Abundant data are available on the ages of perennial seed plants, but we know little about the longevity of ferns. In this study we estimated the ages of 30 colonies of Osmunda claytoniana (Interrupted Fern). Common in the Monongahela National Forest of northeastern West Virginia, Interrupted Ferns typically form convexly bulging, elliptically shaped colonies. We studied colonies larger than 6.6 ft (2 m) in average diameter. Each colony was comprised of dozens of ramets interconnected by a subsurface, dichotomously branching rhizome system. The fronds' stipe bases persist through the colony's life, endure along the entire length of the rhizome, and extend back to the origin of the colony's founding ramet. We estimated the age of a fern colony by dividing the average radius of the colony by the growth rate of its rhizomes. We estimated rhizome growth rate by dividing the average number of stipe bases per length of rhizome by the average number of fronds generated by an individual ramet per year. Rhizome growth rates varied from 0.04 to 0.24 in (0.1–0.6 cm) per year, which represents a slow outward expansion by colonies. Our observational data and derived estimates indicated that these colonies were as old as 414 years. If our estimates are accurate, the Interrupted Fern may be one of the longest-lived organisms in the Appalachian Mountains.