The Guichon batholith in central southern British Columbia is composed of seven granitic phases, together with genetically associated porphyritic dykes and cataclastic breccia bodies. Four of the granitic phases form a concentrically zoned pattern, the geologically youngest phase being in the center and the oldest on the periphery. The batholith as a whole intrudes volcanic rocks of the Upper Triassic Karnian Stage and is unconformably overlain by Middle and Upper Jurassic marine sediments. Twenty-four samples taken for age determination by the potassium–argon method gave an arithmetic mean age of 200 m.y. with a standard error of the mean of ± 5 m.y. No loss of radiogenic argon subsequent to formation of the batholithic rocks is indicated by either geological or analytical observations. No difference of isotopic age could be detected between the geologically oldest and youngest rocks. The batholith originated by a complicated sequence of magmatic evolution, which was completed at a high crustal level during an interval of geologic time too brief to be detected by the analytical technique.