A new species of Mentzelia sect. Bar- tonia (Loasaceae), M. collomiae, is described from central Coconino County, Arizona, U.S.A. It is an edaphic endemic species restricted to cinder areas of the San Francisco Volcanic Field and is distin- guished from other species in the section by seed surface features and by a combination of gross mor- phology, chromosome number, and habitat. Mentzelia sect. Bartonia (Sims) Gilg contains about 35 species of mostly western North American short-lived perennials. Ongoing studies in M. sect. Bartonia and preparation of the Loasaceae treat- ment for the Vascular Plants of Arizona project have confirmed the existence of a new species in Arizona. This taxon was apparently first recognized in the mid 1960s by H. J. Thompson and J. R. Zavortink, as indicated by their annotations of sev- eral specimens of this taxon as "M. volcanicola, n. sp." The same identification was given to other specimens in the mid 1970s in annotations by H. J. Thompson. Since this name was never validly published and since some of the specimens anno- tated as "M. volcanicola" represent M. multiflora (Nuttall) A. Gray, a different epithet is applied to this species.