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Random amplified polymorphic DNAs (RAPDs) and the steam volatile leaf terpenoids of Juniperus erythrocarpa, J. monosperma var. monosperma, J. m. var. gracilis and J. pinchotii were analyzed by principal coordinate analyses and contour mapping. Juniperus monosperma var. gracilis was elevated to specific rank as J. angosturana R. P. Adams nom. nov.; J. erythrocarpa var. erythrocarpa was treated as a segregate of J. pinchotii; and J. erythrocarpa var. coahuilensis was accepted as a distinct species, J. coahuilensis (M. Martinez) Gaussen ex. R. P. Adams. Geographic and chemical analyses of J. coahuilensis suggests that hybridization and introgression with J. angosturana has been factor in the variation of this taxon. RAPDs and terpene data supported the recognition of Arizona-New Mexico populations of J. coahuilensis as a new variety, J. c. var. arizonica R. P. Adams, var. nov. in addition, several terpenoids previously reported as unknown are now identified.