North America, Europe (Russia), Asia.
Genus 1, species 2 (1 in the flora).
The position of Penthorum within Rosales has been disputed extensively. A. Cronquist (1981) considered it to be transitional between Crassulaceae and Saxifragaceae. He included it in Saxifragaceae, stating that Penthorum was not distinct enough from Crassulaceae and Saxifragaceae to warrant being treated as a distinct family. Placement of the genus by others has depended on the morphological, anatomical, and embryological traits emphasized. Molecular studies suggest that the genus is sister to Haloragaceae (D. R. Morgan and D. E. Soltis 1993; D. E. Soltis and P. S. Soltis 1997). Recent authors often have placed it in the monogeneric Penthoraceae.