Phlox idahonis Wherry (Idaho Phlox) is presently a category one candidate for listing under the Endangered Species Act (ESA). This ranking indicates that P. idahonis is critically imperiled due to extreme rarity causing vulnerability to extinction. Previous studies of P. idahonis suggest the species evolved via a vicariant event following the Rocky Mountain orogeny, with the species closest relatives distributed in the eastern and Midwestern United States. However, since it's first collection and subsequent description, the species has received little systematic attention. Additonally, it has been suggested the species is an ornamental Phlox introduced from the eastern United States in the early 1900's. The taxonomic status and evolutionary relationships of P. idahonis to other members of the genus Phlox remain elusive. Phlox idahonis is therefore an excellent example of a species considered for federal ranking before the taxonomic status of the taxon has been definitively established. Our studies reveal that P. idahonis is morphologically allied with P. subsect. Ovatae of the eastern and Midwestern United States. However, the species falls within the range of morphologic variability of P. subsect. Longifoliae, comprising species of western United States distributions. Sequencing of the internal transcribed spacer (ITS) region of rDNA reveals that P. idahonis is most closely related to P. subsect. Longifoliae, notably P. longifolia. These results support P. idahonis as an Idaho endemic and recommend federal listing under the ESA.

Key words: Phlox idahonis, rare Idaho endemic, systematic approaches to conservation