The latest Archean and Proterozoic intrusive mafic rocks (mainly diabase dikes) of the southern Beartooth Mountains were dated by K-Ar and Rb-Sr methods in order to evaluate the relative accuracy and concordance of these methods in rocks of this type. Chemical criteria were used to establish dike sets and these sets show some correlation with age groupings. In general, K-Ar was found to be relatively unreliable for age grouping although useful minimum ages were obtained. Argon loss caused by deformation may be responsible for low K-Ar dates found for older diabase dikes. Rb-Sr dates showed a closer correlation with the chemical groupings. Three ages of mafic intrusion are found in the area: (1) ∼2,550 m.y. (K-Ar and Rb-Sr), (2) ∼1,300 m.y. (K-Ar and Rb-Sr), and (3) ∼740 m.y. (K-Ar).