The life history and population demography of the endangered birdwing pearlymussel (Lemiox rimosus) were studied in the Clinch and Duck rivers, Tennessee. Reproducing populations of L. rimosus now occur only in the Clinch, Duck and Powell rivers, as the species is considered extirpated from the remaining portions of its range in the Tennessee River drainage. Females are long-term winter brooders, typically gravid from Oct. to May. Glochidia are contained in the outer gills and are released in association with a mantle-lure that resembles a small freshwater snail. Estimated fecundity, based on 8 gravid females collected from the Clinch and Duck rivers, ranged from 4132 to 58,700 glochidia/mussel. Seven fish species were tested for suitability as hosts for glochidia, and five darter species were confirmed through induced infestations: Etheostoma blennioides, E. camurum, E. rufilineatum, E. simoterum and E. zonale. Ages of L. rimosus shells were determined by thin-sectioning and ranged from 3 to 15 y in both rivers. Shell growth was higher and maximum size greater in males than females in both rivers. Shell growth was greatest in the Duck River. Densities of L. rimosus in the Clinch River were maintained at seemingly stable but low levels ranging from 0.07 to 0.27 m⁻² from 2004–2007, and in the Duck River at similar but higher levels ranging from 0.6 to 1.0 m⁻² from 2004–2006. In the latter river, abundance has increased since 1988, likely due to improved minimum flows and dissolved oxygen levels in water releases from a reservoir upstream.