Abstract Mesoproterozoic strata from east-central Idaho and the Belt Supergroup of southwest Montana (eight new samples) contain several age groupings of detrital zircon grains: from old to young: a) Laurentian grains older than 1.85 Ga, b) a flood of 1655 to 1790 Ma Paleoproterozoic grains; b) non-North American zircon populations (ca. 1510 to 1625 Ma) with no known source on Laurentia, and c) syn- Belt grains, with groupings at 1480 Ma (syn-lower Prichard) and 1450 Ma (upper Piegan Group). The 1450 Ma = grain age population overlaps a 1454 ± 9 Ma fallout tuff in Glacier National Park. Strata of the Yellowjacket, Hoodoo, Apple Creek, and western Gunsight formations of Idaho all contain the 1450 Ma population, sparse non-North American grains, and dominant Paleoproterozoic populations at 1670 to 1790 Ma. This detrital-zircon signature is comparable to that of the Wallace Formation of the Belt Supergroup. The Swauger Quartzite in Idaho and the eastern Gunsight Formation at the ca. 900 Ma Beaverhead impact site in southwest Montana contain 1710 and 1780 Ma zircon populations identical to those of the Missoula Group of the Belt Supergroup. The E member of the lower Belt Prichard Formation from Plains, Montana, contains a population of syndepositional zircons at 1479 ± 19 Ma. Like the Revett Formation of the Ravalli Group, the E member contains Mesoproterozoic non-North American detrital-zircon populations as well as Paleoproterozoic grains at 1750 to 1790 Ma. The thick east-central Idaho Mesoproterozoic section was deposited after 1450 Ma, with deposition of the 10 km thickness from the lower Yellowjacket Formation through the Gunsight Formation spanning only 10 to 20 My. Given this very high rate of deposition, previous correlations of the Apple Creek Formation with the Piegan Group are permissible if problematic. However, previous correlations of the eastern Gunsight and Swauger formations with the Missoula Group are supported. Their detrital-zircon grain populations are identical.