A new isotopic age of ca. 78 Ma was obtained by the conventional K–Ar method and by laser-fusion 40Ar/39Ar analysis of biotite and sanidine crystals from two bentonites in the Judith River Formation of Kennedy Coulee, Hill County, Montana. The 40Ar/39Ar analyses yielded weighted mean ages of 78.2 ± 0.2 Ma (biotite) and 78.2 ± 0.2 Ma (sanidine) for sample 84MG8-3-4, and 79.5 ± 0.2 Ma (biotite) and 78.5 ± 0.2 Ma (sanidine) for sample 85MG7-16-1. These are the first isotopic age determinations from the Judith River Formation. The dated bentonites in Kennedy Coulee bracket two significant Judithian "age" mammal localities, UCMP V77083 and V81234.The radioisotopic age of ca. 78 Ma for the Hill County local faunas and correlations of other Judithian local faunas with marine units suggest that the duration of the Judithian was at least 5 Ma. Judithian local faunas are probably correlative with the late Campanian of the Western Interior. Similarities in the principal Judithian mammalian faunas from Alberta, Montana, and Wyoming can now be interpreted as documenting a low rate of evolutionary change rather than individual or populational variation of approximately contemporaneous local faunas.