Sr87-Rb87 and Ar40-K40 ages were determined from samples of Precambrian rocks from western Arizona, southern Nevada, and southwestern Utah. These and similar ages measured by other workers from rocks of a geographically contiguous region of California, Nevada, and Arizona and from widely distributed localities in Utah, Arizona, and Sonora, Mexico, indicate that the older Precambrian basement throughout the far southwestern United States was involved in an episode of metamorphism and plutonism 1600–1800 m.y. ago. Well-documented ages of 1350–1450 m.y. are not as extensive as previously thought. An isolated pluton that has an age of 1060 m.y. intruded the older metamorphic terrane in southern Nevada, which indicates that some of the plutonism in the far southwestern United States is later Precambrian.