K-Ar age determinations on muscovite from two granitic bodies provide a minimum date of 480 million years (m.y.) for the end of the late Middle Ordovician. The granitic bodies intruded ultramafic rocks that had intruded the Beauceville Group in the Thetford Mines-Black Lake area of southern Quebec. The Cambrian-Ordovician boundary may be dated at about 535 m.y., 35 m.y. earlier than Kulp's (1961) estimate. It follows that either the estimated duration of the Ordovician (Kulp, 1961) should be lengthened from 75 m.y. to 110 m.y., or the Ordovician-Silurian boundary is also 35 m.y. earlier, that is, about 460 m.y. ago.